A. We use could in a number of ways. Sometimes could is the past of can (see Unit 26C):
* Listen. I can hear something. (now)
* I listened. I could hear something. (past)
But could is not only used in this way. We also in the future (especially to make a suggestion). For example:
* 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 next month, you could stay with Barbara.
* A: If you need money, why don’t you ask Karen?
B: Yes, I suppose I could.
Can is also possible in these sentences (‘We can go for a walk,’ etc.). Could is less sure than can. You must use could (not ‘can’) when you don’t really mean what you say. For example:
* I’m so angry with him. I could kill him! (not ‘I can kill him’)
B. 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 (not ‘It can be Tim’)
In these sentences could is similar to might (see Unit 29-30):
* The phone is ringing. It might be Tim.
C. 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 to New York? You could have stayed with Barbara. (= you had opportunity to stay with her but you didn’t)
* Jack fell off a ladder yesterday but he’s all right. He’s lucky – he could have hurt himself badly. (but ha didn’t hurt himself)
* The situation was bad but it could have been worse.
D. 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 (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.
* 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)