Question

can 和be able to的区别

Answer 1

(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(from www.yygrammar.com)。
(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。

Answer 2

can和be able to 都表示ability，一般可互换，但也有微小差别。
① be able to比can更加正式，因此用的频率低，特别是口语中。
② can的否定can't或cannot (注意不能写成can not), be able to 的否定形式是be not able to或者be unable to
③ be able to有过去、现在、将来时态. 而can只有现在和过去could.
When I was young, I could speak two languages. (过去)
I was able to speak two languages when I was young. (过去)
I can swim, so we should go to the pool. （现在）
I am able to speak two languages (现在)
I will be able to speak two languages sometime in the future, as long as I take languages lessons. （将来）
④ able to可以做状语, 通常用否定形式unable to。
Lisa nodded, unable to speak. Lisa说不出话来，只能点点头。
He lay there, unable to move. 他躺在那里动弹不得。
⑤ 和其他情态动词连用时，只能有be able to，因为两个情态动词不能连用。
You should be able to speak fluent English after the course. 学完这个课程后你应该就能说流利的英语了。
She might be able to speak two languages. 她可能能说两种外语。