Could do and could have done

We use could in a number of ways. Sometimes could is the past of can. We use could to talk about possible actions now or in the future (especially to make a suggestion ):

What shall we do this evening? We could go to the cinema.

Could is less sure than can. You must use could (not ‘can’) when you don’t really mean what you say:

I’m so angry with him. I could kill him! (not ’I can kill him’)

We also use could to say that something is possible now or in the future:

The phone is ringing. It could be Tom.

We use could have done for things which were possible but didn’t happen:

The situation was bad but it could have been worse.

Sometimes could means ‘would be able to…’:

We could go away if we had enough money. ( = we would be able to go away )

I don’t know how you work so hard. I couldn’t do it.

Could have done = would have been able to:

Why didn’t Liz apply for the job? She could have got it.

We could have gone away if we’d had enough money.

The trip was cancelled last week. Paul couldn’t have gone anyway because he was ill. (=he wouldn’t have been able to go)

You did very well to pass the exam. I’m sure I couldn’t have passed it. (=I wouldn’t have been able to pass it if I had taken it)