may 可能..(它的可能性要比might大)
can 能,有能力做某事..(沒有任何推測(cè)的意思.)
he can understand her meaning.他能理解她的意思.
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!/
could 用于虛擬語氣中.或是委婉的語氣中(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)上沒有跟著去.這就是所謂的虛擬語氣.)
could I borrow you pan?(這里就是委婉的語氣,比用can更客氣些.多用于生人之間).
could 也是can的過去式形式.