Main points

* You use `can’ to talk about ability in the present and in the future.

* You use `could’ to talk about ability in the past.

* You use `be able to’ to talk about ability in the present, future, and past.

1 You use `can’ to say that someone has the ability to do something.
You can all read and write.
Anybody can become a qualified teacher.

You use `cannot’ or `can’t’ to say that they do not have the ability to do something.
He cannot dance.

2 When you want to talk about someone’s ability in the past as a result of a skill they had or did not have, you use `could’, `could not’, or `couldn’t’.
He could run faster than anyone else.
A lot of them couldn’t read or write.

3 You also use `be able to’, `not be able to’, and `be unable to’ to talk about someone’s ability to do something, but `can’ and `could’ are more common.
She was able to tie her own shoelaces.
They are not able to run very fast.
Many people were unable to read or write.

4 You use `was able to’ and `were able to’ to say that someone managed to do something in a particular situation in the past.
After treatment he was able to return to work.
The farmers were able to pay the new wages.
We were able to find time to discuss it.

WARNING: You do not normally use `could’ to say that someone managed to do something in a particular situation. However, you can use `could not’ or `couldn’t’ to say that someone did not manage to do something in a particular situation.

We couldn’t stop laughing.
I just couldn’t think of anything to say.

5 When you want to say that someone had the ability to do something in the past, but did not do it, you use `could have’ followed by a past participle.
You could have given it all to me.
You know, she could have done French.

You often use this form when you want to express disapproval about something that was not done.
You could have been a little bit tidier.
You could have told me!

6 You use `could not have’ or `couldn’t have’ followed by a past participle to say that it is not possible that someone had the ability to do something.
I couldn’t have gone with you, because I was in London at the time.
She couldn’t have taken the car, because Jim was using it.

7 In most cases, you can choose to use `can’ or `be able to’. However, you sometimes have to use `be able to’. You have to use `be able to’ if you are using another modal, or if you want to use an `-ing’ form, a past participle, or a `to’-infinitive.
Nobody else will be able to read it.
…the satisfaction of being able to do the job.
I don’t think I’d have been able to get an answer.
You’re foolish to expect to be able to do that.

8 You also use `can’ or `could’ with verbs such as `see’, `hear’, and `smell’ to say that someone is or was aware of something through one of their senses.
I can smell gas.
I can’t see her.
I could see a few stars in the sky.
There was such a noise we couldn’t hear.