We use could in a number of ways. Sometimes could is the past of can:
Listen. I can hear something. (now)
I listened. I could hear something. (past)
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 suggestions).
A : What would you like to do tonight?
B : We could go to the movies.
A : When you go to New York next month, you could stay with Candice.
B : Yes, I guess I could.
Can is also possible in these sentences (We can go to the movies, etc.).
Could is less sure than can.
We also use could (not can) for actions which 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 Candice when I go to New York. (realistic)
Maybe I could stay with Candice when I go to New York. (possible, but less sure)
This is a wonderful place. I could stay here forever. (unrealistic)
We also use could (not can) to say that something is possible now or in the future.
The meaning is similar to might or may:
The story could be true, but I don’t think it is. (not can be true)
I don’t know what time Liz is coming. She could get here at tny time.
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 when you were in New York? You could have stayed with Candice. (you didn’t stay with her)
I didn’t know that you wanted to go to the concert. I could have gotten you a free ticket. (I didn’t get you a ticket)
Dave was lucky. He could have hurt himself when he fell, but he’s OK.
We use couldn’t to say that something would not be possible now:
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 (done):
We had a really good vacation. It couldn’t have been better.
The trip was canceled last week. Paul couldn’t have gone anyway because he was sick. (= it would not have been possible for him to go)