can，
could
表示具有某种能力，指有能力做某事
He could swim when he was five.
By the age of seven he could write beautifully.
表示请求和允许，和may相近；口语中可用could表示委婉语气，但回答时仍用can
—Could /Can I use your pen?
—Yes, of course you can.
Could I ask for your reaction to this policy？
表示推测，常用于否定句和疑问句中
Mike can't have found his car, for he came to work by bus this morning.
I think the work can be completed ahead of time.
表示“有时也会”
It can be quite windy there, especially in spring.
cannot but do sth.表示“只好，不得不”
Father cannot but agree with his daughter's opinion.
can't help doing sth.表示“禁不住做某事”
I can't help feeling sorry for the poor man.
The film is so impressive that we can't help crying.
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