can 能,有能力做某事..(沒(méi)有任何推測(cè)的意思.)
he can understand her meaning.他能理解她的意思.

may/ might 通常用于肯定或否定陳述句中.當(dāng)表示某事可能發(fā)生時(shí),可用could,may,might .如：— Are you coming to Jeff’s party?
你來(lái)參加杰夫的晚會(huì)嗎?
— I’m not sure. I may/ might go to the concert instead.
不太確定,我有可能去參加音樂(lè)會(huì).
may 可能.. (它的可能性要比might大)
could 用于虛擬語(yǔ)氣中.或是委婉的語(yǔ)氣中(希望能得到別人肯定回答的). 和can的意思一樣. "能".
If I could go with you, I should feel very glad.如果我能跟你一起去,我會(huì)很高興的.(但事實(shí)上沒(méi)有跟著去.這就是所謂的虛擬語(yǔ)氣.)
could I borrow you pan? (這里就是委婉的語(yǔ)氣,比用can更客氣些.多用于生人之間).
could 也是can的過(guò)去式形式.
must 一定,是對(duì)某事百分百地肯定..he must have finished homework."他一定做完了作業(yè)"../ tomorrow will must be rainy.明天一定有雨.
must 也有"務(wù)必,必須"的意思.表強(qiáng)制性. you must do it!/