Advanced Class

To talk about the theoretical possibility of something happening we use could, not can. However, we use can, not could, to say that something is possible and actually happens. Compare:

e.g.: It could be expensive to keep a cat. (if we had, it could or it might not be expensive)

e.g.: It can be expensive to keep a cat. ( it can be, and it sometimes is)

We use can't, not couldn't, to say that something is theoretically or actually impossible:

e.g.: There can't be many people in the world who haven't watched television.

e.g.: The doctor can’t see you this morning. He's busy at the hospital.

We use can to indicate that there is a very real possibility of a future event happening. Using could suggest that something is less likely or that there is some doubt about it. compare:

e.g.: We can stay with Jim in Oslo. (We will be able to stay)

e.g.: We could stay with Jim in Oslo (It's possible; if he's there)

To say that someone in the past had general permission to do something - that is, to do it at any time - we can use either could or was/were allowed to. However, to talk about permission for one particular action, we use was/were allowed to, but not could. Compare:

e.g.: Anyone was allowed to fish in the lake. (or could fish ...)

e.g.: Although he didn't have a ticket, Josh was allowed to come in. -( not ... could come in)

In negative sentences, we can use either couldn't or wasn't/weren't allowed to to say that permission was not given in general or particular situation:

e.g.: We weren't allowed to/couldn't open the gifts until that evening.