Main points

* You use `can’ to say that something is possible.

* You use `could’,`might’, and `may’ to indicate that you are not certain whether something is possible, but you think it is.

1 When you want to say that something is possible, you use `can’.
Cooking can be a real pleasure.
In some cases this can cause difficulty.

You use `cannot’ or `can’t’ to say that something is not possible.
This cannot be the answer.
You can’t be serious.

2 When you want to indicate that you are not certain whether something is possible, but you think it is, you use `could’, `might’, or `may’. There is no important difference in meaning between these modals, but `may’ is slightly more formal.
That could be one reason.
He might come.
They may help us.

You can also use `might not’ or `may not’ in this way.
He might not be in England at all.
They may not get a house with central heating.
Note that `could not’ normally refers to ability in the past. See Unit 83.

3 When there is a possibility that something happened in the past, but you are not certain if it actually happened, you use `could have’, `may have’, or `might have’, followed by a past participle.
It could have been tomato soup.
You may have noticed this advertisement.

You can also use `might not have’ or `may not have’ in this way.
He might not have seen me.
They may not have done it.

You use `could not have’ when you want to indicate that it is not possible that something happened.
He didn’t have a boat, so he couldn’t have rowed away.
It couldn’t have been wrong.

You also use `could have’ to say that there was a possibility of something happening in the past, but it did not happen.
It could have been awful. (But it wasn’t awful.)
You could have got a job last year. (But you didn’t get a job.)

4 You also use `might have’ or `could have’ followed by a past participle to say that if a particular thing had happened, then there was a possibility of something else happening.
She said it might have been all right, if the weather had been good. (But the weather wasn’t good, so it wasn’t all right.)
If I’d been there, I could have helped you. (But I wasn’t there, so I couldn’t help you.)

5 `Be able to’, `not be able to’, and `be unable to’ are sometimes used instead of `can’ and `cannot’, for example after another modal, or when you want to use a `to’-infinitive, an `-ing’ form, or a past participle.
When will I be able to pick them up?
He had been unable to get a ticket.

6 You use `used to be able to’ to say that something was possible in the past, but is not possible now.
Everyone used to be able to have free eye tests.
You used to be able to buy cigarettes in packs of five.

7 Note that you also use `could’ followed by a negative word and the comparative form of an adjective to emphasize a quality that someone or something has. For example, if you say `I couldn’t be happier’, you mean that you are very happy indeed and cannot imagine being happier than you are now.
You couldn’t be more wrong.
He could hardly have felt more ashamed of himself.