Compare can/could etc. for actions:
can
- I can go out tonight. (= there is nothing to stop me)
- I can't go out tonight.
could
- I could go out tonight, but I'm not very keen.
- I couldn't go out last night. (= I wasn't able)
|
can or may
- Can I go out tonight? (= do you allow me?)
- May I go out tonight? (= do you allow me?)
|
will/won't
- I think I'll go out tonight.
- I promise I won't go out.
would
- I would go out tonight, but I have too much to do.
- I promised I wouldn't go out.
|
shall
- Shall I go out tonight? (do you think it is a good idea?)
|
should or ought to
- I should go out tonight. (= it would be a good thing to do)
- I ought to go out tonight. (= it would be a good thing to do)
|
must
- I must go out tonight. (= it is necessary)
- I mustn't go out tonight. (= it is necessary that I do not go out)
needn't
- I needn't go out tonight. (= it is not necessary)
|
Compare could have ... / would have ... etc. :
could
- I could have gone out last night, but I decided to stay at home.
would
- I would have gone out last night, but I had too much to do.
should or ought to
- I should have gone out last night. I'm sorry I didn't.
- I ought to have gone out last night. I'm sorry I didn't.
needn't
- I needn't have gone out last night. (= I went out, but it was not necessary)
|
We use will/would/ may etc. to say whether something is possible, impossible, probable, certain etc. Compare:
will
- 'What time will she be here?' 'She'll be here soon.'
would
- She would be here now, but she's been delayed.
|
should or ought to
- She should be here soon. (= I expect she w1ll be here soon)
- She ought to be here soon. (= I expect she w1ll be here soon)
|
may or might or could
- She may be here now. I'm not sure. (= it's possible that she is here)
- She might be here now. I'm not sure. (= it's possible that she is here)
- She could be here now. I'm not sure. (= it's possible that she is here)
|
must
- She must be here. I saw her come in.
can't
- She can't possibly be here. I know for certain that she's away on holiday.
|
Compare would have ... / should have ... etc.:
will
- She will have arrived by now. (= before now)
would
- She would have arrived earlier, but she was delayed.
|
should or ought to
- I wonder where she is. She should have arrived by now.
- I wonder where she is. She ought to have arrived by now.
|
may or might or could
- She may have arrived. I'm not sure. (= it's possible that she has arrived)
- She might have arrived. I'm not sure. (= it's possible that she has arrived)
- She could have arrived. I'm not sure. (= it's possible that she has arrived)
|
must
- She must have arrived by now. (= I'm sure - there is no other possibility)
can't
- She can't possibly have arrived yet. It's much too early. (= it's impossible)
|
