26 Can/could和（be ）able to

We use can to say that something is possible or that somebody has the ability to do something. We use can +infinitive（ Can do /can see etc.）
-We can see the lake from our bedroom window.
-Can you speak any foreign languages?
-I can come and see you tomorrow if you like.

The negative is can't（=can not）
-I'm afraid I can't come to the party on Friday.

（Be） able to… is possible instead of can, but can is more usual:
- Are you able to speak any foreign languages?

But can has only two forms can（ present） and could（ past）. So sometimes it is necessary to use be able to:
I can't sleep.
but
I haven't been able to sleep recently.（ can has no present perfect）

Tom can come tomorrow.
But
Tom might be able to come tomorrow.（ Can has no infinitive）

Could and was able to…
Sometimes could is the past of can. We use could especially with:
see, hear, smell, taste, feel, remember, understand
-When we went into the house, we could smell burning.
-She spoke in a very low voice, but I could understand what she said.
-We also use could to say that somebody has the general ability or permission to do something:
-My grandfather could speak five languages. We were completely free. We could do what we wanted（=we were allowed to do...）
We use could for general ability. But if we are talking about what happened in a particular situation, we use was/ were able to...or managed to...（not could）:
-The fire spread through the building quickly but everybody was able to escape. or everybody managed to escape. （but not 'could escape'）
- They didn't want to come with us at first but we managed to persuade them. or ...we were able to persuade them, （but not 'could persuade'）
Compare:
-Jack was an excellent tennis player. He could beat anybody.（=He had the general ability to beat anybody）
but
-Jack and Alf had a game of tennis yesterday. Alf played very well but in the end jack managed to beat him. or ...was able to beat him. （=he managed to beat him in this particular game）

The negative couldn't （could not） is possible in all situations:
- My grandfather couldn't swim.
- We tried hard, but we couldn't persuade them to come with us.
- Alf played well but he couldn't beat Jack.