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.