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Give me 31 minutes.
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I will make PyTorch click forever.
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If PyTorch has ever felt intimidating, don't worry, you're not alone.
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Every training script has these five lines.
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Predict, measure the loss, compute the gradients, update the weights, and clear the gradients.
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So what does backward really compute?
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What does step change?
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And why clear anything at all? But it's actually simple.
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We'll walk through one tiny real example,
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predicting what a New York taxi ride costs from 36 real trips first by hand then with these five lines
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and watch it get close to the taxi meters own rate by itself by the end
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you'll have mastered all five lines in 31 minutes we'll understand this whole board one stop at a time tensors
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and what we can do with them how a model predicts
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and measures its loss gradient descent by hand autograd and finally the professional loop let's get started
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The first stop is Tensors, and to build one, we first need some data.
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We'll use one real data set the whole way.
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New York yellow taxi rides from the city's public trip records.
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On three days in January 2024, a Monday, a Wednesday, and a Saturday, we took the first ordinary ride in every two -hour slot.
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For each one, we wrote down what the meter charged.
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Look at the boxed row.
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It's Monday's noon ride, 1 .38 miles for $11 .40, and we'll follow it all the way through.
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How do we hold these fares?
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We use a tensor, which is just a grid of numbers.
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It can have as many axes as we need, one for a row, two for a table like ours.
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Look at the code.
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We give Torch .Tensor our list of rows, one row per day.
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We get back a grid with a shape, 3x12.
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But which of those two numbers, 3 and 12, is which?
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Each direction of the grid is called an axis or a dimension, DIM for short.
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PyTorch counts them from zero.
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DIM zero runs down the days and there are three.
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DIM one runs across the two hour slots and there are twelve.
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The shape gives the size of each DIM.
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And when there's no data yet, how do we make a tensor of some shape?
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Look at the code.
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Torch .zeros, 2, 3, gives a grid full of zeros.
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The 2 and the 3 are its shape.
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2 along dim 0, so 2 rows, and 3 along dim 1, so 3 columns.
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Torch .ones, 2, 3 makes a grid of that shape full of ones.
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What if we want a shape full of some other value?
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Torch .full takes the shape as one tuple, and then the value.
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Here's 7.
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And torch .i makes a 3x3 grid with 1s on the diagonal and 0s everywhere else.
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Let's make random numbers next, the kind a model starts from.
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Torch .rand draws each one between 0 and 1.
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Torch .rand draws them around 0 from a bell curve.
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And what about counting?
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Torch .a range of 12 counts from 0 up to 11, just like Python's range.
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The end, 12, is left out.
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The main point.
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A tensor is a grid of numbers and its shape says how many there are along each dim.
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That's the first stop done.
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Our fares sit in a tensor and we can build new ones from a shape.
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On to the second stop, tensor operations.
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What can we do with our table of fares?
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Here are 12 of them, running on our table at once.
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Each one answers a question about the rides.
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Don't try to read them all now.
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We'll go through the main ones together, one at a time.
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How do we get one day's rides?
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We pick by index, like in a Python list.
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Look at the code.
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Fares of 0 is row 0, which is Monday's 12 fares.
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Picking one day drops the day axis, so the shape is just 12.
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What about one time slot on every day?
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Inside the brackets, a comma separates the axes.
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First dim 0, the days, then dim 1, the slots.
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A colon on its own means all of that axis.
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So fares of colon 6 is every day, slot 6.
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Slots start at midnight and last two hours each, so slot 6 is the noon ride.
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Next, let's take the first few slots.
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A slice takes a range and keeps the axis.
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So fares of colon, colon three, is every day's first three slots from midnight to six in the morning.
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That's a three by three grid.
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Which rides cost more than $30?
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Look at the code.
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Fares greater than 30 gives a grid of true and false, shaped like fares.
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It's called a Boolean mask, and we'll call it pricey.
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Index with it, and we can pull out just the true cells.
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That's five rides over $30.
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What if every ride were capped at $30?
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Torch .where of pricey, 30, and fares checks the mask sell by sell.
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It puts 30 where the mask is true.
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Everywhere else it keeps the original fare.
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What's the average fare each day?
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The days run down dim zero, so let's try fares .mean with dim equals zero first.
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Pause and guess.
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Will we get three numbers or 12?
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Watch what happens.
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It's 12, one average per time slot, and we wanted one per day.
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Why 12?
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This is important.
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The DIM you pass is the axis that gets aggregated away.
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Aggregating means combining many numbers into one, like taking their average.
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So DIM equals zero average the days away, and the slot stayed.
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We want the slots gone, so we pass DIM one.
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Look at the code.
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Fares .mean with dim equals 1 averages each day's 12 fares into one number.
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That's about $19 .63 on Monday, $14 .32 on Wednesday, and $16 .24 on Saturday.
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What was the priciest ride each day?
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Max also takes dim equals 1, but it gives back two tensors, the highest fare and the index of its slot.
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On Monday, it's $45 .70 at 10 in the morning.
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On Wednesday, it's $44 .30 at 4 in the morning.
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And on Saturday, it's $33 .80 at 6 in the morning.
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What about the three priciest rides each day?
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Top K of 3 gives the three largest in order and their slots.
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On Monday, that's $45 .70, $43 .60, and $38 in slots 5, 2, and 1.
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Here, Dim 1 isn't aggregated away.
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It shrinks from 12 to 3.
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Pop quiz.
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Fares has shape 3 by 12.
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What shape does fares .max with dim equals 0 dot values have?
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Is it A, 3, 1 per day?
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B, 12, 1 per slot?
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C, 3 by 12, the whole grid?
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Or D, just one number?
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Pause the video here and pick one.
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The answer is B.
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The DIM you pass is the axis that gets aggregated away.
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DIM 0 runs down the days, so max aggregates the days away and keeps the highest fare in each of the 12 slots.
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Choice A is what DIM equals 1 gives, one number per day.
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Choice C keeps both axes, but max with a DIM always aggregates one away.
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Choice D is fares dot max with no DIM at all, the single highest fare.
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How do we get one row per time slot instead of one per day?
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Dot T for transpose swaps the two axes, so the 12 slots become 12 rows with one column per day.
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Let's see each day in quarters.
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Reshape 3, 4, 3 keeps the 36 numbers but gives them three axes.
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That's three days along dim 0, four quarters of the day along dim 1, and three slots in each quarter along Dim 2.
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Now a mean over Dim 2 averages the slots away, leaving each quarter's average.
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On Monday, the night rides averaged about $32 and the evening ones were under $10.
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And Flatten goes the other way.
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It puts all 36 fares in one long row.
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Monday first, then Wednesday, then Saturday.
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We'll need it in a minute.
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You'll meet plenty more in real code, so here are the everyday ones on one card run on a small 2x3 grid.
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Feel free to pause.
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Some in argmax work like mean and max.
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Each one aggregates an axis away, and the at sign is the matrix multiply rows times columns.
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Here it multiplies x by its transpose x .t, which we just met, and star multiplies cell by cell.
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And here are a few more.
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View changes the shape, like reshape does, and permute reorders the axes, like .t does.
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Unsqueeze and squeeze add and drop an axis of size 1.
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Gather picks one cell per row by index and clamp clips every number into a range.
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The main point?
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We pick numbers by their index and in mean, max or sum, the DIM we pass gets aggregated away.
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We have 36 taxi rides.
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How does a model learn from them?
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That's the second stop done.
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Operations pick, combine and rearrange the tensor's numbers.
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A model learns by guessing first, so the third stop is predicting.
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We'll pair each ride's miles with its fare and fit a line.
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The model should guess what every rider wants to know.
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How much will this ride cost?
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Each ride's record also says how far it went.
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So we have a second table of miles shaped like the fares.
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Flatten both and we get 36 pairs, miles in, fare out.
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We'll call them M and F.
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How do we see M and F as a table of pairs?
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Look at the code.
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Torch .StackWithDim equals 1 puts the two rows side by side along a new axis.
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That gives 36 rows of miles and fares.
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Torch .Cat would glue them end to end instead into one row of 72.
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So how much should a ride cost?
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New York's rate card has an answer.
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It's $3 to start, plus 70 cents every fifth of a mile.
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A mile is five fifths, so that's 350 a mile.
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Can a model find a rule like that from our 36 rides alone?
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We know the meter's rule so we can check.
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Almost every ride sits above the cards line, and we'll see why.
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Let's keep the model as simple as it gets.
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It's a line with two numbers to learn.
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The first is a weight, W, the price per mile, which sets how steep the line is.
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The second is a bias, B, the price to start, which lifts it.
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Its output, f -hat, is the predicted fare.
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From here on, we'll call all the numbers a model learns its weights.
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PyTorch calls them parameters.
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A line like this is called linear regression.
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We start from a lazy guess.
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W equals 1 and B equals 0.
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That's a dollar a mile and nothing to start.
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Let's work out one prediction by hand.
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Monday's noon ride went 1 .38 miles.
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So the guess is 1 times 1 .38 plus 0, which is $1 .38.
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The meter charged $11 .40.
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The miss is the guess minus the fare, so it's 1 .38 minus 11 .4.
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That's minus 10 .02, about $10 short.
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What about every other ride?
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One line, W times M plus B, does that for all 36 rides at once.
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All 36 miles times W plus B give 36 predictions?
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PyTorch reuses the 1W and the 1B for every ride, and that's called broadcasting.
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The math writes a prediction as F -hat, and the code calls it Pred.
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The main point, a model turns an input into a prediction, and ours turns each ride's miles into a guess at its fare.
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Our lazy line now predicts all 36 fares, but every guess is too low.
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Fourth stop, the loss function.
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How wrong is the prediction?
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Each prediction misses by some amount.
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So how do we turn 36 misses into one number?
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The naive tries to just average them.
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Right now, every prediction is below its point, so nothing cancels yet.
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But watch what happens once the line runs through the points.
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Misses above and below cancel out and even a bad line could average zero, so we square each miss instead.
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Monday's noon miss is minus 10 .02.
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Squared, that's 100 .4.
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Square all 36 misses the same way.
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Add up the 36 squares and we get 9 ,432 .56.
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Divide by 36 and we get 262 .02.
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Pause here if you want to check the division.
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That's the mean squared error.
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The lazy line's loss is big because it misses by a lot.
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The main point, the loss is one number that says how wrong the model is.
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Pop quiz.
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One line misses one of our 36 rides by $6.
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Another misses 3 by $2 each. Which loss is bigger?
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Is it a, the one with the single $6 miss?
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B, the one with the three $2 dollar misses?
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C. They're equal? Or D.
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It depends on the fares?
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Pause the video here and pick one.
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The answer is A.
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Squaring makes one big miss count far more than several small ones.
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One six dollar miss squares to six times six, 36, while three two dollar misses square to four each,
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and 4 plus 4 plus 4 is 12.
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Choice B looks fair since both lines are off by $6 in total, but its squares add up to just 12.
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Choice C would be true if we added the misses without squaring them.
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Choice D is wrong because only the misses go into the loss.
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Which way should W and B move to lower the loss?
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Our loss squares the misses and averages them into one number.
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Its derivatives say which way to move W and B, and they lead to the fifth stop, gradient descent.
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Compute the gradient, then step against it.
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What is the gradient?
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It's the derivatives of the loss, one per weight.
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DL by DW is how fast the loss changes when only W moves and B holds still.
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That's the slope of the loss along W, DL by DB does that for B.
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Two weights, so the gradient is two numbers.
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How do we compute DL by DW?
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and dl by db.
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Let's write down what we have.
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Each prediction is w times m plus b, and the loss is the mean of the squared misses.
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The derivative of x squared is 2 times x, so each squared miss has derivative 2 times the miss.
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Moving w by 1 moves each prediction by its miles, m.
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Moving b by 1 moves it by 1.
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Multiply those together and then average over the 36 rides.
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That's the chain rule, the one bit of calculus here.
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You don't need to master it because soon Autograd does it for us.
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Let's work out both derivatives for one ride by hand.
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Take Monday's noon ride.
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For W, its term is 2 times minus 10 point.
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O, 2 times 1 .38.
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That's minus 27 .66.
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For B, there are no miles.
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It's just 2 times minus 10 .02, which is minus 20 .04.
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Now average those over all 36 rides.
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That gives DL by DW equals minus 126 .21, and DL by DB equals minus 27 .56.
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Pause here if you want to check the arithmetic.
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Which way should we move?
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Look at this map of the loss.
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W runs across and B runs up.
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Each curve is one loss level, and the star is the bottom.
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We can only draw this map because there are two numbers.
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A big model has millions, so we follow the gradient instead.
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At our start, both derivatives are negative, so the loss goes down if both numbers go up.
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How far do we step?
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The naive tries to move W by its whole derivative.
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Watch what happens.
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It jumps past 120, much too far.
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So we scale the gradient by a small learning rate, here 0 .01.
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This is important.
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We step against the gradient.
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Our new W is 1 minus 0 .01 times minus 126 .21.
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That's 2 .262.
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And B becomes 0 minus 0 .01 times minus 27 .56, or about 0 .276.
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The line swings up toward the points.
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Here's one whole gradient descent step with every variable on screen, one row per ride.
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Each row has the miles, the fare, the prediction, the miss, its square, and the miss times the miles.
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The squares average to the loss.
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Average the misses times the miles and double it, and that's dL by dw.
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Average the misses alone and double it, and that's dL by db.
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And the last line moves both.
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Feel free to pause and check any row.
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What happens if we repeat that step 100 times, still with a learning rate of 0 .01?
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W shoots up early.
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It's overshooting to make up for the missing B.
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But B crawls.
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Its derivative is much smaller because moving B by 1 changes each miss by just 1.
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Moving W by 1 changes it by the ride's miles.
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After 100 steps, B is only at 3 .71.
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That's still well short of where it ends up.
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It does get there.
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It just needs more steps and later will meet an optimizer that fixes this crawl.
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So let gradient descent run a thousand times.
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The line settles on the points and the loss falls from 262 .02 to 10 .15.
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W is 3 .76 and B is 5 .66.
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That's about 376 a mile and 566 to start.
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The meters card says 350 a mile so that's close.
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The start is higher and part of the reason is on that card too.
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In slow traffic, the meter charges by the minute, so slow rides cost more than their miles alone.
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The main point?
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To train a model, we predict, measure the loss, compute the gradient, step, and repeat.
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Pop quiz.
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A ride's prediction is too high, so its miss is positive.
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Which way does that ride push W?
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Is it a up, B, down, C, not at all?
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Or D, only B moves?
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Pause the video here and pick one.
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The answer is B.
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We step against the gradient, so too high means come down.
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That ride's term in DL by DW is 2 times the miss times the miles, a positive number, so W goes down.
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Choice A is what happens at our lazy start, where every ride is too low, so their terms are negative, and W goes up.
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Choice C would need a miss of 0.
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Choice D is wrong, because that term, two times the miss times the miles, isn't zero.
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So W moves two.
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But who works out the gradient for a model with a million weights?
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By hand, we worked out the gradient and stepped against it.
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A thousand times.
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Now the sixth stop?
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Autograd.
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PyTorch finds the gradients for us.
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How does Autograd find the gradients?
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Look at the code.
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We create W and B with requires grad equals true, so PyTorch knows we'll want their gradients.
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From then on, it keeps recording everything we compute from them.
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Each operation becomes a node in a graph.
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The graph is a record of how the loss was made.
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Here it's a multiply, an add, a subtract, a power for the square, and a mean.
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PyTorch names each node for the step it will run on the way back, like mole backward.
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And each result's gradfn, its gradient function, remembers the node that made it.
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Then loss .backward walks that graph backward, from the loss to W and B.
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It fills W .grad with DL by DW minus 126 .21, and B .grad with DL by DB minus 27 .56.
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That's exactly what we worked out by hand.
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On the way back, each node multiplies its own local derivative into the gradient.
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Take Monday's noon ride again, and leave out the means 1 over 36 for now.
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Its factors multiply to minus 27 .66 for W and minus 20 .04 for B.
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That's what we got by hand.
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Every ride has its own product and the mean averages them.
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It's the chain rule done for you.
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In our back propagation video, we take it apart step by step.
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In a training loop, backward runs every step.
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So what happens if we compute the loss and call backward again?
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Does the new gradient replace the old one or add to it?
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Vali .grad doesn't stay at minus 126 .21.
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It doubles to minus 252 .42.
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This is important.
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Backward adds into .grad instead of replacing it.
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PyTorch's documentation says it plainly.
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Backward accumulates gradients, so you might need to zero them.
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That's why every training step clears .grad once the update is done.
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Then the next step doesn't add on top of old gradients.
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Our code clears them by setting both to none.
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Now, let's run one fresh backward and use its gradients in our loop.
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There's one rule.
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The update isn't part of the model's math, so it shouldn't be recorded.
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Watch what happens if we update anyway.
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Changing the recorded tensor in place means overwriting its own numbers, and that raises an error.
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So we do the update inside Torch .nograd.
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Let's put it together.
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Here's our loop with Autograd.
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Compute the loss, call backward, step every weight inside NoGrad, and clear the gradients for the next step.
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And its printout matches our loop by hand.
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Autograd records every operation, then walks the record backward to fill DotGrad.
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Our loop now gets its gradients from Autograd.
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And the last stop?
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The Professional loop our training, organized with torch .optim and nn .module, where nn stands for neural network.
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Our loop still updates and clears each weight by hand.
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In PyTorch, that's the job of an optimizer.
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It's an object that holds the weights and steps them for us.
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The simplest is torch .optim .sgd plane gradient descent.
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So its numbers match ours, crawl and all.
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Faster optimizers come later.
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Look at the code.
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We give it the weights and the learning rate.
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SGD stands for Stochastic Gradient Descent, named for the random batches we'll meet at the end.
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Opt .step updates every weight, Opt .0Grad clears every dot grad, and the loop prints those four rows again.
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But real models can have millions of weights.
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So how do we hand them all to the optimizer?
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Look at the code.
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We subclass nn .module, which is PyTorch's base class for models.
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Ours is called fair model.
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We store each weight on self as an nn .parameter.
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That's a tensor with requires grad already on.
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Then model .parameters finds them all for you.
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How does the model make a prediction?
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Look at forward.
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It says how an input becomes an output, it, and calling model of m runs it.
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All 36 miles go in and 36 predictions come out.
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That's what nn .module is, a container that finds every weight for you, even inside layers nested in it.
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What's a layer?
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It's a building block with its own weights, like nn .linear, which will open up soon.
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A bigger model, two layers, nest two of them, and nn .module still finds all 13 parameters.
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Every model also gets the tools on this card.
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It can list its weights by name, count them, and save them.
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Our mean squared error is built in too.
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NN .MSELoss makes a loss function.
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You call it on the prediction and the target, which is the real fare.
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It gives 262 .02 again, and it's the one our training loop will use.
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Here are the five lines from the start, and now you know everyone.
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Predict.
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Measure the loss with nn .mse loss.
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Compute the gradients with backward, update the weights with step, and clear the gradients with zero grade.
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Pop quiz!
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What goes wrong if you delete opti .0 grad from the five lines?
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Is it A.
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Nothing changes?
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B. The gradients pile up, so the steps grow and overshoot?
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C. Backward raises an error? Or D.
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The model forgets its weights?
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Pause the video here and pick one.
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The answer is B.
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Backward adds into .grad.
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Without 0 grad, step 2's .grad holds step 1's gradient plus its own, and the old ones never get cleared.
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So old gradients keep pushing, and W shoots past the bottom, to over 8 within 5 steps.
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Choice A would only be true if backward replaced .grad instead of adding to it.
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Choice C is wrong because PyTorch is happy to keep adding.
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Choice D is wrong because zero grad clears dot grad, never the weights.
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Side by side with our loop by hand, you can see every job line up.
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The five lines just hand the gradients and the step to PyTorch.
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Starting from w equals 1 and b equals 0, both land on 3 .76 and 5 .66.
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Our model is trained, so what's next?
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Let's use it, save it, and then train it on batches.
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To use it, we turn recording off with Torch .noGrad since we won't call backward.
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Its exact weights are 3 .7556 a mile and 5 .658 to start.
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With those it says a one mile ride costs about $9 .41.
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Five miles cost $24 .44 and 10 miles cost $43 .21.
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So before you get in, you know roughly what you'll pay.
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How do we save the trained model?
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State Dyke holds the trained weights by name.
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Torch .save writes it to a file.
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Then a fresh fair model which starts at 1 and 0 loads them back.
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Before batches there's one more piece.
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PyTorch has our model built in.
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Look at the code.
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NN .Linear is a weight times the input, plus a bias.
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That's one number in and one number out.
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Let's hand it our row of 36 numbers.
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Watch what happens.
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It fails.
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It reads the last axis as one rides inputs.
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So our row of 36 looks like one ride with 36 inputs.
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It wants one input per ride.
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How do we fix that shape error?
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We add a new last axis with M of colon, none.
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That turns the row into a column of 36 examples, each one number -wide.
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Inside NN .linear is a matrix multiply.
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The 36 by 1 column times the 1 by 1 weight, plus the bias, gives 36 predictions.
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Our five lines train it from small random starting numbers to 3 .76 and 5 .66 again.
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With those, Monday's noon row is 1 .38 times 3 .7556 plus 5 .658, which is $10 .84.
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What if a dataset is too big to use all at once?
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A data loader deals it out in shuffled batches.
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Look at the code.
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A tensor dataset joins each ride's miles to its fare, like our stack from earlier.
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Then the loader deals the 36 pairs out, 12 at a time.
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Our five lines run once per batch, on its 12 miles X and 12 fares Y.
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One trip through all three batches is called an epoch, so 300 epochs make 900 steps, and they land near 3 .75 and 5 .69.
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Those random batches are the stochastic and stochastic gradient descent.
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Predict measure the loss backward step clear.
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That's the five -line loop.
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What else can we choose?
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Three parts of the loop have a menu.
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The optimizer, the model, and the loss.
459
We'll go through them fast.
460
First, the optimizer, which is where we fix the crawl.
461
At our learning rate, plain SGD needs 551 steps to settle because B crawls.
462
Momentum keeps some speed from recent steps and lets older ones fade, so B's small pushes add up and build speed,
463
and Momentum needs just 100 steps.
464
Atom gives each weight its own step size, so even B's small derivative gets a full -size step, and Atom needs just 88.
465
Here are 8 on our loss, each at a learning rate tuned for it.
466
Each badge counts its steps to the star, so feel free to pause here.
467
Switching is one line and the loop doesn't change.
468
Next we can change the model itself.
469
A linear layer only makes straight lines.
470
But some data is curved like how fast taxis move at each time of day.
471
Quick at night, slow at midday.
472
Let's put an activation, a small function applied to every number between two layers.
473
Now the line can bend.
474
Without one, two linear layers in a row still make a straight line.
475
Relu zeros the negatives, and Gelu is a smooth Relu.
476
Sigmoid squeezes every number between 0 and 1, and Tan does it between minus 1 and 1.
477
And Softmax turns a row into probabilities.
478
Let's try an activation on real data.
479
Take the average taxi speed in each 2 -hour slot from the trip records again.
480
Then give a model the time of day and ask it to predict all three days' speeds.
481
Straight lines miss by up to 7 .1 miles an hour.
482
With one ReLU in the middle, the worst miss drops to 1 .62.
483
Last, the loss is a choice too.
484
Mean squared error is for numbers like ours.
485
L1 is for when a big miss shouldn't count extra and Huber mixes the two.
486
For picking a category, there's cross entropy, or binary cross entropy for yes or no. Let's try them on two rides.
487
Our trained line misses Monday noon by minus 0 .56, and Monday's 10 o 'clock ride by minus 13 .23.
488
Mean squared error squares them.
489
The big miss counts about 175, the small one about a third, average 87 .67.
490
L1 takes their plane sizes, average 6 .90 Huber takes half the small miss squared
491
And the big one's plane size, less a half, so it's 6 .44 For numbers, swapping one in is a single line
492
For categories, cross -entropy grades made up scores for
493
which day a ride was on And binary cross -entropy grades a yes or no
494
Is it over $30?
495
And that's the whole loop Tensors hold the numbers
496
The law says how wrong we are Autograd finds the gradients.
497
Gradient descent moves the weights, and the professional loop does it in five lines.
498
Now PyTorch clicks.
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Vocabulário e dicas de fala para esta lição
Esta aula de conversação usa o vídeo “Give me 31 minutes, I will make PyTorch click forever”. As palavras que mais se repetem: ride, gradient, fare, step, mile. Este vídeo tem 498 frases e 4730 palavras para praticar shadowing. A fala dura 31:50. O falante mantém um ritmo estável de cerca de 149 palavras por minuto, confortável para o shadowing. Apenas 77% das palavras estão entre as 3.000 mais comuns do inglês, por isso o vocabulário é exigente.
Vocabulário principal deste vídeo
15 palavras do vídeo que vale a pena aprender, com pronúncia e significado:
| Palavra | Pronúncia | Significado |
|---|---|---|
| fare substantivo | /ˈfɛə/ | tarifa |
| minus | /ˈmaɪ.nəs/ | menos |
| slot substantivo | /slɑt/ | fenda, ranhura |
| backward adjetivo | /ˈbækwɚd/ | de volta, atrás |
| loop substantivo | /luːp/ | laço |
| torch substantivo | /ˈtɔːtʃ/ | tocha |
| prediction substantivo | /pɹɪˈdɪkʃən/ | predição, previsão |
| grid substantivo | /ɡɹɪd/ | matriz, quadriculado |
| axis substantivo | /ˈæksɪs/ | eixo |
| predict verbo | /pɹɪˈdɪkt/ | prever |
| pause substantivo | /pɔːz/ | pausa |
| derivative substantivo | /dɪˈɹɪvətɪv/ | derivado |
| noon substantivo | /nun/ | meio-dia |
| aggregate substantivo | /ˈæɡ.ɹɪ.ɡət/ | agregado |
| descent substantivo | /dəˈsɛnt/ | descida, descimento |
Phrasal verbs que você vai ouvir
| Palavra | Pronúncia | Significado |
|---|---|---|
| work out verbo | /ˌwɝk ˈaʊt/ | calcular |
| run down verbo | atropelar | |
| go down verbo | descer | |
| go up verbo | subir | |
| run across verbo | passar correndo | |
| write down verbo | anotar | |
| come out verbo | /ˌkʌm ˈaʊt/ | sair, partir |
| end up verbo | terminar, acabar |
Frases que vale a pena repetir
Frases curtas e completas do vídeo que você pode usar na conversa do dia a dia:
- How do we hold these fares?
- What can we do with our table of fares?
- How do we get one day's rides?
- Which way should we move?
- Our model is trained, so what's next?
Gramática neste vídeo
As estruturas que o falante mais usa, com as palavras exatas do vídeo:
| Estrutura | No vídeo |
|---|---|
| Voz passiva be + particípio passado — o foco está no que acontece, não em quem faz | is called · is left · were capped |
| Orações relativas who / which + oração — informação extra sobre uma pessoa ou coisa | tensor, which is · module, which is · linear, which will |
Pronúncia para ficar de olho
O falante usa 31 contrações e formas reduzidas, como we'll, doesn't, don't. Diga-as na forma curta, do jeito que você ouve.
- Os sons de “th”: mathematics /mæθ(.ə)ˈmæt.ɪks/, smooth /smuːð/, sixth /sɪksθ/, gather /ˈɡæðɚ/, arithmetic /æɹɪθˈmɛtɪk/
- Os sons de “sh” e “zh”: prediction /pɹɪˈdɪkʃən/, activation /ˌæktəˈveɪʃən/, dimension /daɪˈmɛn.ʃən/, documentation /ˌdɑkjəmɛnˈteɪʃən/, overshoot /ˌoʊ.vɚˈʃut/
- Palavras longas — acerte a sílaba tônica: derivative /dɪˈɹɪvətɪv/, parameter /pəˈɹæm.ə.tɚ/, mathematics /mæθ(.ə)ˈmæt.ɪks/, activation /ˌæktəˈveɪʃən/, ordinary /ˈɔɹdɪnɛɹi/
Sons difíceis para falantes de português:
- Consoante final — sem acrescentar um “i” depois: slot /slɑt/, backward /ˈbækwɚd/, loop /luːp/, grid /ɡɹɪd/, predict /pɹɪˈdɪkt/
- /l/ final — a língua toca o céu da boca, não vira “u”: crawl /kɹɔːl/, module /ˈmɑd͡ʒul/, settle /ˈsɛtəl/, pile /paɪl/, neural /ˈnʊɹəl/
- s + consoante no início — sem um “i” antes: slot /slɑt/, smooth /smuːð/, script /skɹɪpt/, swing /ˈswɪŋ/, stack /stæk/
Como praticar com este vídeo
- Ouça o vídeo inteiro uma vez sem falar e anote as palavras que você não conhece.
- Faça shadowing frase por frase na velocidade normal, repetindo cada uma até o seu ritmo ficar igual ao do falante.
- Grave a sua voz e compare com o original, prestando atenção a palavras como fare, minus, slot.
O que é a Técnica de Shadowing?
Shadowing é uma técnica de aprendizado de idiomas com base científica, originalmente desenvolvida para o treinamento de intérpretes profissionais. O método é simples, mas poderoso: você ouve áudio em inglês nativo e repete imediatamente em voz alta — como uma sombra seguindo o falante com 1-2 segundos de atraso. Pesquisas mostram melhora significativa na precisão da pronúncia, entonação, ritmo, sons conectados, compreensão auditiva e fluência na fala.