We use can(do) to say that something is posible or that someone has the ability to do something.
| -|- | I can speak Chinese. |
| <-| | I could speak Chinese when I was a kid. |
| |-> | I will be able to speak Chinese by the time I finish my course. |
We often use can or could when we ask someone to do something.
| -|- | Could you tell me how to get to the station? |
We sometimes use can when we offer to do things.
| -|- | Can I help you? |
If we do something during a specific event , we have to use was/were/will able to
| -|- | With a burst of adrenaline, people can pick up cars.. |
| <-| | With a sudden burst of adrenaline, he was able to lift the car off the child's leg. |
| |-> | With a sudden burst of adrenaline, he will be able to lift the car. |
We sometimes use could to talk about possible future actions, especially when we make suggestions.
| |-> | We could go to the cinema. |
We use could have done to say that we had the ability or the opportunity to do something but did not do it .
| <-| | We didn't go out last night. We could have gone to the cinema but we decided to stay at home. |
We use couldn't have done to say that we wouldn't have been able to do it if we had wanted or tried to do it.
| <-| | The football match was cancelled last week. Tom couldn't have played anyway because he was ill. |
We use could have done to talk about the past.
| <-| | I was so tired. I could have slept for a week. |