Page 66 - Basic English Usage

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Probability
We do not usually use can when we are talking about the chances that
something is true, or that something will happen. For this idea
(probability), we prefer could, may or might (see 199).
‘Where’s Sarah?’ ‘She may/could be at Joe's place.’
(NOT +Ske-eanbe ... ')
We may go camping this summer. (NOT -We-earrge ... )

could have ...
We use a special structure to say that something was possible, but did
not happen.

|
| could have + past participle

That was a bad place to go skiing — you could have broken your (eg.
Why did you throw the bottle out of the window? It could have hit
somebody.

80 can: permission, offers, requests and orders

Permission

We use can to ask for and give permission.
‘Can | ask you something?’ ‘Yes, of course you ean.’
Can! have some more tea? You can go now if you want to.
We also use could to ask for permission. This is more polite or formal.
Could | ask you something, if you're not too busy?
May and might are also possible in formal and polite requests for
permission. (See 200.)
May | have some more tea?

Past permission
We use could to say that we had ‘general’ permission to do something
at any time.
When | was a child, | could watch TV whenever | wanted to.
But we don't use could to talk about permission for one particular past
action.
| was allowed to see her yesterday evening. (NOT teeutdsee . . .)
(This is like the difference between cou/d and was able to. See 78.3.)

Offers
We use can when we offer to do things for people.
‘Can! carry your bag?’ ‘Oh, thanks very much.’
‘lean baby-sit for you this evening if you like.’ ‘No, it's all right, thanks.’