must 表示必须、一定时,Must...肯定回答:Yes,...must.否定回答：No,...needn't.（特别要注意否定回答不能用mustn't.因为needn't 意思是不需要,mustn't是不允许.例如：Must I borrow the books with ID card 我必须用身份证借书吗?No,you needn't.不,你不需要.如果是用No,you mustn't.就变成 “不,你不允许.”了!
may 表示请求,可以时,May...肯定回答：Yes,...may.\ Yes,please.\Certainly\ Sure...
否定回答：No,...can't.\ I'm sorry ...can't.\ I'm afraid...can't.
need 表示需要、必须时,Need ...肯定回答：Yes,...must.否定回答：No,...needn't.( 特别要注意肯定回答时用...must做答.）