Model Verb

can, be able to

can表示某事是可能的，某人有某种能力

  • Natasha can play the piano.

或者有某种机会

  • She can go to the music room.

be able to更正式一些，但以下情况必须用be able to:

  1. 带to的不定式
    • It's nice to be able to go to the opera.
  2. 在情态动词后
    • Melanie might be able to help us.
  3. 现在完成时
    • It's been quiet today. I have been able to get some work done.

我们还可以用can, will be able to表示将来:

  • If we earn some money, we can go/we will be able to go on holiday next summer.

could, was/were able to

表示过去的能力或机会

  • Natasha could play/was able to play the piano when she was four.

但它们之间也有区别，当表示这种能力或机会导致了某个具体的动作时，即实际发生了某事，只能用 was/were able to.

  • The plane was able to take off at eleven o'clock, after the fog had lifted.
  • The children could swim when they were quite young.(过去的能力)
  • The children were able to swim when they were quite young.(过去的动作)

同感官动词和表示思维活动的动词连用时，通常用could，而不用was/were able to.

  • We could see the village in the distance.
  • I couldn't understand what was happening.

许可(can, could, may)

can, could, may都可以用来表示请求许可，could比较客气，而may比较正式。

请求

  • Can I use your pen?
  • Could we borrow your ladder, please?
  • May I see the letter?

许可、不许可

表示许可时，用can和may，但不用could. May比较正式，在口语中不常用。

  • Could I borrow your calculator? ~ Of course you can.

类似的，表示不许可时，用can't或may not，也可用must not，但不用could not。

谈论他人的许可

在谈论他人制定的规定时，用can谈论现在, could谈论过去, be allowed to(现在、将来、过去).

  • Each passenger can take one bag onto the plane.
  • In the 1920s you could drive without taking a test.
  • Will I be allowed to record the interview on tape?

could和be allowed to都可以表示过去的许可，但如果许可导致了某个具体的动作时，即 实际发生了某事时，只能用was/were allowed to，而不能用could.

  • I was allowed to leave work early yesterday.

可能和肯定: may, might, could, must

may, might表示某事很有可能发生, could表示某事时有可能的，但可能性比较小。

  • It may/might be a bomb.
  • You could win a million pounds!

may, might, could后可以用动词的进行时(be + -ing形式)

  • That man may/might be watching us.

may not, might not表示反面情况是有可能的, could not表示某事是不可能的.

当意识到某事肯定属实时，用must；当意识到某事不可能时，用can't.

must, have to

must, have to表示某事是必须的.

  • must表示一种吩咐, 也可以表示一种愿望
    • You must exercise.(我在吩咐你)
    • I must buy a newspaper. I want to see the racing results.
  • have to表示客观情况
    • I have to exercise.(可能是医生要求我)

在过去时或带will的将来时的场合中，需要用have to.

  • Emma had to go to the dentist yesterday.
  • That wasn't very good. We will have to do better next time.

在一些其它结构中也需要用have to:

  1. 带to的不定式
    • I don't want to have to wait in a queue for ages.
  2. 在情态动词后
    • Emma has toothache. She might have to go to the dentist.
  3. 现在完成时
    • Did you have to pay for your second cup of coffee?

have got to

与have to意思相同, 不过have got to是非正式的用法, 主要用于现在时.

  • I have to/I've got to make my sandwiches.
  • Do we have to apply/Have we got to apply for a visa?