Unit 27. Could (do) and could have (done)

We use could in a number of ways. Sometimes could is the past of can.

But could is not only used in this way. We also use could to talk about possible actions now or in the future (especially to make a suggestion)

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

It's a nice day. We could go for a walk.
When you go to New York nex month, you could stay with Barbara.

Can is also possible in these sentences. 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 Tim.
• I don't know when they'll be here. They could arrive at any time.

Can is not possible in these examples.

In these sentences could is similar to might

• The phone is ringing. It might be Tim.

Compare could (do) and could have (done)

• I'm so tired. I could sleep for a week. (now)
• I was so tired. I could have slept for a week (past)

Most often, we use could have (done) for things which were possible but did not happen.

• Why did you stay at a hotel when you went to New Work?
  You could have stayed with Barbara.
• 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.
• I don't know how you work so hard. I couldn't do it.

Could have (done) = would have been able to (do)

• Why didn't Liz apply for the job? She could have got it.
• We could have gone away if we'd had enough money.