can 和be able to 都可以表示能力.
can泛指一般的能力,而且只有两种形式,
即:can, could．be able to 则主要指具体做到了某件事的能力,其形式主要是靠be 发生变化,所以形式比can 多．
可以说：I can swim. I am able to swim.
但是不能说：All the people could escape from the big fire in time. 只能说：All the people were able to escape from the fire in time.
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