1)can could 表示能力;可能 , 只用于现在式和过去式。
be able to可以用于各种时态。
They will be able to tell you the news soon. 他很快就能告诉你消息了。
2)只用be able to
a. 位于助动词后。
b. 情态动词后。
c. 表示过去某时刻动作时。
d. 用于句首表示条件。
e. 表示成功地做了某事时,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.
注意:could不表示时态
1)提出委婉的请求,。
--- Could I have the television on?
--- Yes, you can. / No, you cant.
2)在否定,疑问句中表示推测或怀疑。
He couldnt be a bad man.
他不大可能是坏人。
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