can和be able to的区别

1.can表示“能力”时，和be able to相当，许多场合都可以互相替换。但当叙述过去经过一番努力才能完成的事情或前面有特殊说明，表示你有能力时，只能用be able to。
【正】Can you speak any foreign languages?
你会说外语吗？
【正】Are you able to speak any foreign languages?
你会说外语吗？
【误The fire spread the building quickly but everybody could escape.
【正The fire spread the building quickly but everybody was able to escape.
大火迅速蔓延到整幢大楼，但大家都逃了出来。
【正The fire spread the building quickly but everybody managed to escape.
大火迅速蔓延到整幢大楼，但大家都想法逃了出来。

2.be able to 比can有更多形式。
No one could answer the question.
没人能回答这个问题。（这里could可用was able to 代替）
When he grows up, he will be able to support his family.
他长大后就能养家了。
Frank is ill. He hasn't been able to go to school for one week.
弗兰克病了，已经一周没去上学了。
I'm sorry for not being able to help you in time.
对不起，不能及时帮你的忙。

3.could经常和动词see, hear, smell, taste, feel, remember, understand等连用。
When we went into the house, we could smell something burning.
当我们走进屋子时，我们闻到什么东西烧焦了。（不用was able to）
She spoke in a very low voice, but I could understand what she said.
虽然她讲话的声音很低，但我还是明白了她说的话。

4.在谈论说话时发生的动作，用can，不用be able to。
【误】Look! I'm able to swim.
【正】Look! I can swim. 看，我会游泳了！

5.在用used to谈论过去用能力做某事，但现在失去了这个能力时，我们用be able to。
如：I used to be able to play the violin.

6.在谈论将来有能力做某事。我们用will be able to。
如：After only a few lessons, you will be able to understand basic Spanish.

7.在其它动词may,might,would,want或hope后，用be able to.
如：He might be able to fix your car.
You should be able to taste the difference.
I want her to be able to use a computer.

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