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.

情态动词的回答