can，could和be able to的区别

Question

如题所述

Answer 1

1、时态不同

从时态形式来看，can 只有现在式和过去式(could)，而 be able to 则可根据需要使用多种时态，还可以和一些情态动词连用，或用作动词不定式等非谓语形式。could是can 的过去式。

I haven't been able to sleep recently. 最近我睡不着觉。

He can swim. 他会游泳。

He couldn't be a bad man.他不大可能是坏人。

2、含义不同

用于现在时，can 泛指一般的能力，而 be able to 则主要指具体做某件事的能力。could不表示时态。

He can swim. 他会游泳。

I am able to express my idea in a clear way. 我能清楚地表达自己的想法。

Could I have the television on?你能打开电视吗？

3、表能力时不同

如果要表示"一时的能力"，即"在过去某时有某种能力，并顺利地完成某事"，则要用 was / were able to，而不能用 could。could 表示可能或有能力做。

Though he was sick, he was able to swim across the river. 虽然他病了，但他还是游过了河。

I could pass the examination. 我当时有能力通过考试。（有能力，但实际上不一定做得到）

Answer 2

1. can 既表示现的“能力”，也能表示将来的“能力”。但表示现在的能力通常用can，表示将来的能力，一般用will/shall be able to。
I can’t do it now, but I can do it later.
Fire cannot destroy gold.
I'll be able to speak German in another two years．

2. be able to可以用于各种时态, 也可用于情态动词后：is/ are able to, was/ were able to, will / shall be ablte to , ought to be able to, might be able to.
They will be able to tell you the news soon. 他很快就能告诉你消息了。
I shan’t be able to come on Janury 1.
I haven’t been able to get in touch with her.
I ought to be able to live on my salary.

4. could表示过去经常性或一般性的能力.
My father could speak ten languages．
At that time I could still read without spectacles.

5. 表示过去一次性的或经过一番努力, 终于能够成功地完成某一具体动作时, 只能用was/were able to, 不能用could。
He was able to flee Europe before the war broke out.
= He managed to flee Europe before the war broke out.
= He succeeded in fleeing Europe before the war broke out.
I was able to finish my homework in an hour．

在否定句中没有此限制：
I couldn’t / wasn’t able to play the piano when I was at school.
He got so drunk that he couldn’t / wasn’t able to find the front door.

6. can/ could 的主语既可是人，又可是物； be able to 只用于有生命的名词、代词作主语的句子中。
It can be referred that…