Can is the Present Simple. Could is the Past Simple. Can borrows the rest of tenses from be able to.

 

MODAL

VERBS

                                                                                           

 

 

 

ABILITY

Can (less formal and more usual) – ability in the present or future

Be able to (more formal) – ability in the future

I can swim well.

Ican help you next week. (usual)

I will be able to help you next week. (less usual)

Could (more usual) – ability in the past (repeated actions, !!! with the verbs see, hear, smell, understand, guess, remember – for single actions)

Was able to (= managed to, less usual) – ability in the past (repeated and single actions)

Both could and was able to can be used in negative sentences and questions for either repeated or single actions.

She could play the piano when she was 5. (repeated action)

I was able to finish my project yesterday. (single action)

 

She couldn’t / wasn’t able to pass her exam. (single)

Could / Were you able to have a rest yesterday? (single)

Can is the Present Simple. Could is the Past Simple. Can borrows the rest of tenses from be able to.

I haven’t been able to finish it yet.

 

 

POSSIBILITY

can + present infinitive– general possibility – it is theoretically possible

may / might / could + present infinitive– perhaps; it is likely; it’s possible to happen in the future or perhaps it’s true at the moment

may / might / could + perfect infinitive – perhaps it happened in the past

could + perfect infinitive - it was possible but didn’t happen

This road can get very busy.

She may / might / could be late.

He may / might / could have broken his computer.

 

You could have hit that man.

PROBABILITY

ought to / should + present infinitive –probably;it is probable now or in the future

ought to / should + perfect infinitive – we expected it to happen but we don’t know if it happened or not

Tom ought to / should get this job.

They ought to / should have come two hours ago.

LOGICAL ASSUMPTIONS

can't / couldn’t + present infinitive(I don’t think; it’s logically improbable)

must + present infinitive (I think, I’m sure, it’s logically probable)

can’t– opposite of must(I’m sure it isn’t true)

can’t / couldn’t + perfect infinitive (it’s impossible that it happened in the past)

must + perfect infinitive (it’s very probable that it happened in the past)

He can’t be poor. His house is very big and modern.

You are a headmaster. You must have a lot of duties.

It can’t be true.

They can’t have forgotten about our dinner; they must have missed the bus.

To express possibility in questions we use: