| 释义 |
Can and could Both these verbs indicate ability in some respect. The use of could is usual in clauses that contain a reference to past time.- Morag can speak French quite well now.
- I couldn’t play chess two years ago, but I can now.
- When I was younger I could play tennis really well.
- Winston is so strong he can lift me right off my feet.
- Can you get up the stairs without help?
- You can come over for dinner whenever you like.
Can and could are used:- to indicate that you know how to do something.
- Mary can do these sums.
- I couldn’t draw very well when I was younger.
- to show ability to do something. (Compared with be able to, can indicates ability of a more general nature that includes ‘is permitted to’.)
- When I was younger I could ski really well.
- Graham can run ten miles in 25 minutes.
- Are you able to walk to the car?
- to make polite requests or to ask for permission:
Could is more tentative than can. (Compare with may, which is more formal.)
- Can I borrow the car tomorrow evening, Mum?
- Could I come with you on the trip?
- May I take this book home with me?
- to express the possibility of an action in the future, especially when the possibility is related to plans or projects. (Compare with may, where the possibility referred to is still uncertain and in the future.)
- We can go to Paris next week since you are free.
- We could go to Paris next week if you are free.
- We may go to Paris, but it depends on our finances.
- to express the possibility of an action in the present.
- You can dive off these rocks; it is quite safe here.
- We could dive off the rocks, but we must take care.
- to talk about actions that were possible but did not happen, using could + the perfect form of have.
- Mary could have stopped the fight but she didn’t.
- using the perfect form of have, to speculate about actions that have recently taken place.
- Who could/can have broken the window?
- Who would have guessed that they were related?
A distinction between can and could is observed in conditionals. Could is used when the conditions are not met.- If Louisa is coming, she can look after the children for a while.
- If Helen had more money, she could buy a computer.
When changing sentences from direct to reported speech can is usually changed to could.- Bernard said, ‘I can do it for you, Sue.’
- Bernard said that he could do it for Sue.
| can | | The negative form is: | cannot. |
- I cannot understand why he did it.
| The contracted negative form is: | can’t. |
| could | | The contracted negative form is: | couldn’t. |
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