I can speak English. Or I am able to speak English.
The basic structure of “to be able to”
subject + be able to + verb
I am not able to see the sea from this window.
The differences between can and to be able to
“Can” has only two forms: peresent (can) and past (could)
“be able to” is possible in all tenses:
Present simple: I am able to drive.
Past simple: I was able to drive when I was 13.
Future simple: I will be able to drive one day.
Perfect tenses: I have been able to drive since I was 12.
The differences between could and to be able to
If you mean that someone managed to do something in one particular situation, you have to use was/were able to
John was an excellent tennis player . He could beat anybody.(= He had the ability to beat anybody.)
But once he had a difficult game against George. George played very well but in the end John was able to beat him. (= He managed to beat him in this particular game.)
The negative couldn't is possible in all situations.