O verbo be able to é geralmente uma alternativa do modal can/could.
I can go later.
I’m able to go later.
I couldn’t bring it.
I wasn’t able to bring it.
No passado, was able to pode ter um outro significado, diferente de could, ou seja, estar apto a conseguir fazer alguma coisa em uma determinada situação, por exemplo:
I was able to convince her to do it.
She was able to catch him on the way out.
I wasn’t able to talk to him before he left.
Para expressar possibilidades futuras, não se deve usar can e sim be able to.
We’ll be able to fly to New York if they get ready for the flight.
Mike won’t be able to come if you hold the party while he is travelling.
Ao contrário de can, podemos usar be able to como um particípio com verbos modais.
I might be able to help you soon.
It’s nice being able to work at home sometimes.
I’ve been able to relax a bit as we’ve had much less work.