义务：must / have to / need to

表示必须做某事有三种说法：must、have to、need to——后面都接动词原形。Must 是情态动词；have to 和 need to 的用法和普通动词一样。

💡Must 永远不加 -s、不加 to、不用 do：She must go。Have to / need to 正好相反：She has to go。Does she need to go?

1. have to——因为规定、工作或客观情况而必须（是事实，不是说话人的个人看法）

• I have to wear a uniform at work.

  我上班必须穿制服。

• Linh has to take two buses to get to school.

  阿玲上学得坐两趟公交车。

• Do you have to work on Sundays?

  你星期天也得上班吗？

2. must——说话人自己觉得必须：提醒自己，或者强烈推荐

• It's late. I must go.

  很晚了。我得走了。

• I must call my grandma this weekend.

  这个周末我一定要给奶奶打个电话。

• You must try the coconut coffee here — it's amazing!

  你一定要尝尝这里的椰子咖啡——太好喝了！

3. must——书面规定、标识和说明

• All visitors must sign in at the front desk.

  所有访客都必须在前台登记。

• Passengers must keep their bags with them at all times.

  乘客必须随时看管好自己的行李。

4. need to——为了某个目的或因为当前的情况而需要做某事

• My phone is almost dead. I need to charge it.

  我手机快没电了。我得充一下电。

• Do we need to book a table, or can we just walk in?

  我们需要订位吗，还是直接去就行？

5. have got to——和 have to 一样，在非正式口语中非常常见

• Sorry, I've got to go — my bus is leaving.

  抱歉，我得走了——我的公交车要开了。

💡在美式英语中，表示义务的 must 听起来相当强硬、正式；美国人通常说 have to 或 have got to（gotta）。拿不准的时候，用 have to 几乎总没错。

Must 只有一种形式，只能谈论现在或将来。表示过去、用在 will / might 后面、用于现在完成时或放在另一个动词后面时，要用 have to。

• You mustn't touch the paintings.

  请勿触摸画作。

• You don't have to take your shoes off. Come in!

  你不用脱鞋。进来吧！

💡mustn't = 禁止；don't have to = 可做可不做。不要把两者弄混。另见“无需：don't have to / needn't”一课。

• ✗You must to turn off your phone in class.

  ✓You must turn off your phone in class.

  must 后面不加 to。

• ✗She have to finish her homework.

  ✓She has to finish her homework.

  he/she/it → has to。

• ✗Last week I must work every evening.

  ✓Last week I had to work every evening.

  表示过去用 had to。

• ✗Do you must wear a helmet?

  ✓Do you have to wear a helmet?

  must 不和 do 连用；要问 Do you have to…?

• ✗We will must show our passports.

  ✓We will have to show our passports.

  将来：will have to。

• ✗She needs wear a uniform.

  ✓She needs to wear a uniform.

  need 作普通动词 → need to + 动词。