一、表示“能够做某事”，强调通过努力而获得的能力。
She is able to play the piano.
她能够弹钢琴。
I am able to drive the truck.
我能够开那辆卡车。
She is able to speak three languages.
她能够讲三种语言。
二、be able to 后面接动词原形，表示能够做某事
He is able to drive a car.
他会开车。
They are able to sing the song.
他们能够唱这首歌。
三、be able to 的时态
一般现在时：am/is/are able to
I am able to run very fast.
我能够跑得非常快。
一般过去时：was/were able to
He was able to make lots of friends when he was young.
当他年轻的时候，他能够交很多朋友。
一般将来时：will be able to
They will be able to finish the work soon.
他们很快就能够完成工作。
现在完成时：have/has been able to
I wonder whether they have been able to find jobs.
我想知道他们是否（已）能够找到工作。
Many women have been able to pick up the threads of their former career.
许多女性得以重操旧业。
四、be able to 的否定式
一般现在时：am not/isn’t/aren’t able to
I'm not able to understand this math problem.
我不能理解这道数学题。
一般过去时：wasn’t/weren’t able to
My legs failed me and I wasn't able to run the last lap.
我这腿不争气，最后一圈跑不动了。
一般将来时：won’t be able to
Jack won't be able to come for another three days.
杰克再过三天就不能来了。
现在完成时：haven’t /hasn’t been able to
We haven't been able to get in touch with them yet.
我们还没能与他们取得联系。
五、be able to 的同义词
was/were able to ≈ could
am/is/are able to ≈ can
wasn’t/ weren’t able to ≈ couldn’t （最常用）
am not/ isn’t /aren’t able to ≈can’t
be able to 的否定式与 be unable to 同义
比如：
Many families on low incomes will be unable to afford to buy their own home. = Many families on low incomes will not be able to afford to buy their own home.
许多低收入家庭将会买不起自己的房子。