(1) be able to 强调通过努力而获得的能力，而can则强调自身已具有的能力。如：

She can sing the song in English. 她能用英语唱这首歌。

He will be able to sing this song in English in a few minutes, too. 几小时之后，他也能用英语唱这首歌。

(2) be able to 强调一种结果，而can只强调一种可能。如：

Luckily, he was able to escape from the big fire in the end. 幸运的是，他终于逃出了大火。

If he got here a few hours earlier, I could save him. 要是他早几小时来，我还能救他。

(3) be able to 可以有各种时态；而can只有一般现在和一般过去两种时态。如：

I could help you last night, but you didn’t come. 昨天晚上我能帮你，而你又没来。

Can you see it there? 你能看见它在那儿吗？

He is / was / will be able to help you. 他能帮你的忙。

(4) can可用于表示可能性，推测，允许等情况，而be able to通常不这样用。