Could (do) and could have (done)

Learn and practise the use of could (do) and could have (done).

A Sometimes could is the past of can:

  • Listen. I can hear something. (now)
  • I listened. I could hear something. (past)

But could is not always past. We also use could for possible actions now or in the future, especially to make suggestions. For example:

  • a: What shall we do tonight?
    b: We could go to the cinema.
  • a: When you go to Paris next month, you could stay with Sarah.
    b: Yes, I suppose I could.

Can is also possible in these sentences (‘We can go to the cinema.’ etc.). Could is less sure than can.

B We also use could (not can) for actions that are not realistic. For example:

  • I’m so tired, I could sleep for a week. (not I can sleep for a week)

Compare can and could:

  • I can stay with Sarah when I go to Paris. (realistic)
  • Maybe I could stay with Sarah when I go to Paris. (possible, but less sure)
  • This is a wonderful place. I could stay here for ever. (unrealistic)
C We also use could (not can) to say that something is possible now or in the future:

  • The story could be true, but I don’t think it is. (not can be true)
  • I don’t know what time Lisa is coming. She could get here at any time.

Compare can and could:

  • The weather can change very quickly in the mountains. (in general)
  • The weather is nice now, but it could change later. (the weather now, not in general)
D We use could have (done) to talk about the past. Compare:

  • I’m so tired, I could sleep for a week. (now)
  • I was so tired, I could have slept for a week. (past)
  • The situation is bad, but it could be worse. (now)
  • The situation was bad, but it could have been worse. (past)

Something could have happened = it was possible, but did not happen:

  • Why did you stay at a hotel? You could have stayed with me.
    • David was lucky. He could have hurt himself when he fell, but he’s all right.
E I couldn’t do something = it would not be possible:

  • I couldn’t live in a big city. I’d hate it. (= it wouldn’t be possible for me)
  • Everything is fine right now. Things couldn’t be better.

For the past we use couldn’t have … (= would not have been possible):

  • We had a really good holiday. It couldn’t have been better.

Note that ‘I couldn’t do something’ has two meanings:
(1) I couldn’t = it would not be possible now, I would not be able:

  • I couldn’t run ten kilometres now. I’m not fit enough. (= I would not be able)

(2) I couldn’t = I was not able (past)

  • I couldn’t run yesterday because I’d hurt my leg. (= I was not able)