CAN

*something that is happening as we speak

*before passives

*meaning is “know how to”

In other options we can use can or be able to, but in the examples above can is more natural.

BE ABLE TO

*single achievement in the past

COULD (more natural use than be able to)

*general ability in the past

*negative sentences in the past

*with verbs of the senses and “thinking” (decide, believe, remember, understand)

*after the only thing/place/time and after all (when it means the only thing)

*with almost, hardly, just, nearly

+possibility:

It could be expensive to keep a cat = if we had one

It can be expensive to keep a cat = we have one

We can stay with Jim in Oslo = we will be able to stay

We could stay with Jim in Oslo = it’s possible, if he’s there

+permission

could or was/were allowed to – general permission

e.g. Anyone could/was allowed to fish in the lake when the council owned it.

was/were allowed to – permission for one particular past action

e.g. Although he didn’t have a ticket, Ken was allowed to come in.

couldn’t or wasn’t/weren’t allowed to - in negative sentences (general/particular)