如：
1)―Must I finish my homework before eight o'clock? ―Yes, you must.
2)―Must I attend the meeting?
―No, you needn't(No, you don't have to). You can askTomto go instead.
3)―May I watch TV after supper? ―Yes, you may. / No, you mustn't.
4)―May I stop here? ―No, you mustn't.
5)―Need we stay behind after school? ―Yes, you must.
6)―Need he go now? ―No, he needn't.
表達(dá)必須,一定的意思的一般現(xiàn)在時(shí)的句子中只能用must（需要注意的是：除了現(xiàn)在一般時(shí),其他各種時(shí)態(tài)都要借用have to）