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.
表達必須,一定的意思的一般現在時的句子中只能用must（需要注意的是：除了現在一般時,其他各種時態都要借用have to）