1、 在回答must开头的一般疑问句时，肯定回答用must，否定回答用needn't或don't have
to. 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 ask Tom to go instead.

2、在回答may开头的一般疑问句时，肯定回答用may，否定回答用mustn't. 如：
1)―May I watch TV after supper? ―Yes, you may. / No, you mustn't.
2)―May I stop here? ―No, you mustn't.

3、在回答need开头的一般疑问句时，肯定回答用must，否定回答用needn't. 如：
1)―Need we stay behind after school? ―Yes, you must.
2)―Need he go now? ―No, he needn't.

表达必须,一定的意思的一般现在时的句子中只能用must（需要注意的是：除了现在一般时,其他各种时态都要借用have to）