be able to.
1、当can ( could ）作“有能力、能够”的意思时，可以和 be able to 互换。如：He will come if he can. =He will come if he is able to.
I can see you tomorrow. =I will be able to see you tomorrow.
2、can ( could ）表示“请求别人的许可”，这时不可以和 be able to 换用。
Mom ， can I watch some TV
Could you please turn it a little down ?
3、 can ( could ）表示“可能性” 也不可以用 be able to 替代。如：My mom can't be at home at this time .
4、在表示“某个动作被成功完成、某个计划被成功实施”时，只能用 was able to ( be able to 的过去式），不可以用 could 。如：He was able to go to London yesterday and he enjoyed himself very much .
