Sometimes could is the past of can (see Unit 26):

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

• 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.

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)

• 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)

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.

• 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)

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)

• 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.

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.

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