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.
could 是can的过去式