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.