For / since / ago

For, since and ago all talk about time, but in three different ways:

Example: today is Thursday, and Lan arrived in Hanoi on Monday. You can say the same thing in three ways:

• Lan has been in Hanoi for three days.

• Lan has been in Hanoi since Monday.

• Lan arrived in Hanoi three days ago.

💡Two common traps: “starting in 2019 and still going” is since 2019 — from usually needs to/until after it (from 2019 to 2022); “two years back from now” is two years ago, not before two years.

For answers How long? and works with any tense, not only the present perfect.

• I've had this phone for three years.

  I still have it

• We lived in Hue for two years, and then we moved to Saigon.

  finished → past simple

• I'm staying with my cousin for a week.

• In present perfect negatives: I haven't seen her for years. American English also often uses in: I haven't seen her in years.
• for + a length of time (for two hours) is different from during + an event or period (during the film, during the summer): during tells you when, not how long.
• for ages (informal) = for a very long time: I haven't been to the cinema for ages.

Since gives the starting point — a day, a year, an event — and the action continues from then until now. So the main clause is usually in a perfect tense (have/has + V3 or have/has been + -ing).

• My dad has worked at the hospital since 2015.

• I haven't eaten anything since this morning.

• Binh has played football since he was little.

  since + a past simple clause

• It's been two years since I last saw Minh.

The pattern It's been (= It has been) + a length of time + since + past simple is very common in speech. British English also says It's two years since… with the same meaning.

Ago comes after the length of time and counts back from now. It points to a finished moment, so it goes with the past simple, not the present perfect. To ask: How long ago…? or When…?

• Hung moved to Hanoi three years ago.

• How long ago did you start this job? — About six months ago.

• I called her ten minutes ago, but she didn't answer.

💡Ago counts back from now; before counts back from a past moment: She left Hanoi in 2020. She had moved there two years before (= in 2018). On its own, before means “at an earlier time”: I've never been here before.

Choose the tense by asking yourself: is it still going on, or finished?

You can change the same information from one pattern to another:

• I bought this bike two years ago. → I've had it for two years.

  bought → have had, not have bought

• Minh started learning the guitar in 2023. → He's been learning it since 2023.

• The last time I saw Hoa was in May. → I haven't seen Hoa since May.

• ✗I have studied English since five years.

  ✓I have studied English for five years.

  five years is a length of time → for.

• ✗I have lived here for 2019.

  ✓I have lived here since 2019.

  2019 is a starting point → since.

• ✗We met each other before two years.

  ✓We met each other two years ago.

  “Back from now” = a length of time + ago.

• ✗I have seen that film two weeks ago.

  ✓I saw that film two weeks ago.

  ago points to a finished time → past simple.

• ✗I live here since 2020.

  ✓I have lived here since 2020.

  since needs a perfect tense in the main clause.

• ✗I'm waiting for an hour!

  ✓I've been waiting for an hour!

  To say how long up to now, use a perfect tense, not the present continuous.