Can, Could, Be able to

modals

You use CAN to describe an ability in the present.

Annie can swim, but she can't ride a bike.

I can play the guitar and sing.

You can also use BE ABLE TO to describe an ability in the present.

He's able to park a car, but he's not able to drive in traffic yet.

He’s crazy. He thinks he is able to stop time with his mind.

You use CAN to describe an ability in the present.

Annie can swim, but she can't ride a bike.

I can play the guitar and sing.

You can also use BE ABLE TO to describe an ability in the present.

He's able to park a car, but he's not able to drive in traffic yet.

He’s crazy. He thinks he is able to stop time with his mind.

In the PRESENT, the only difference between CAN and BE ABLE TO, is that

CAN is more common and BE ABLE TO is more formal.

use COULD to describe a general ability in the past.

My father could speak five languages.

He was a very good tennis player. He could beat anybody.

use WAS/ WERE ABLE TO if you are talking about what happened in a particular situation.

Jack and John played a tennis match yesterday and John was able to beat Jack.

There was a fire, but we were able to scape.

But in the negatiave, you can use COULDN’T in both cases:

My father couldn’t swim.

I couldn’t read when I was 5.

There was a big fire and some people coulnd’t escape.

Yesterday Jack couldn’t beat John.

For forms and tenses other than present or past, you have to use BE ABLE TO.

We haven't been able to play tennis for a long time. (Present Perfect)

I won’t be able to be there tomorrow. (Future)

I’ve always wanted to be able to dance. (To-infinitive)

etc.