can/could/be able to後面須加原形動詞。
例句:
Henry can speaks three languages. (X)
Henry can speak three languages. = Henry is able to speak three languages. (O)

be able to的 be動詞須注意配合時態與主詞單複數變化。
例句:
The kid isn’t able to swim last year, but he swims very well now. (X)
The kid wasn’t able to swim last year, but he swims very well now. (O)

過去式 could 與 was/were able to意思有差異。 could 表示「可能」或「可以」做某事，但未必真正做到， was/were able to則表示當時確實有做到。
例句:
It wasn’t an easy match, but Sean could beat the opponent finally. (X)
It wasn’t an easy match, but Sean was able to beat the opponent finally. (O)

can/could 的時態只有現在與過去式，不可與其它助動詞連用。因此，如須使用未來式或其他時態則須以「助動詞 +be able to」表示。
例句:
I’m afraid I won’t can meet you tomorrow. (X)
I’m afraid I won’t be able to meet you tomorrow. (O) 

否定句中， can/could後面加 not，表示「不行」、「無法」或「不可能」。 be able to則在 be動詞後面加 not，表示能力做不到。
例句:
The baby doesn’t be able to walk yet. (X)
The baby can’t walk yet. = The baby isn’t able to walk yet. (O)