must:
肯定must（必須）,否定must not/mustn't/needn't（不準,禁止）He mustn't be there 他一定不在那兒.Mustn't I smoke here?禁止在這兒吸煙嗎?you mustn't smoke here.你不能在這兒吸煙.
注意,must 的否定也有用needn't的.Must I finish the work this morning?Yes,you must/No,you needn't.我必須早上就完成工作嗎?是的,必須/不,不需要.
need:
肯定need(需要/必須),否定needn't.You need not go yet ,need you?Yes,I must/ I need to/ I have to/No,I needn't或者 No,I don't have to.你不一定要去,是嗎?是的,我一定要/必須/不得不去/不,不一定要去.
shall(只和第一人稱連用 we/I):
肯定shall（將/必將/可以/.好嗎）,否定 shall not.注意,當回答中主語不再是第一人稱,Shall要變成should.Shall I go to school?Good idea/Yes,you should./No,you shouldn't.但是回答主語仍舊是第一人稱時,shall 不用變.Shall we go to school?Yes,we shall/No,we shall not/I'd love to,but I can't.
may:
肯定may(可以/許可/能),否定may not.注意過去式,虛擬語氣或客氣是用might/mightn't.May I close the window?Yes,go ahead/Yes,you can（一般不這么說）/No,I'm afraid you may not.
can：
肯定can（可以/許可/能）,否定can‘t這個你應(yīng)該懂的.恩謝謝很詳細><那么may I go shopping with you ?用why not?還是Yes，you can.泥..我在糾結(jié)因為你說yes，you can一般不這么說是不是不算口語或者什么呢。根據(jù)西方人的習慣，一般用why not？恩謝謝很詳細><對了我看到別的網(wǎng)頁上有這樣總結(jié)的你看看對不對↓↓may--- Yes, you may. No, you can't/mustn't(禁止）. 也有說用may not的。need --- Yes, you must. No, you needn't.must-- Yes, you must. No, you needn't.can -- Yes, you can. No, you can't.其實you may和why not都可以的，看習慣而已