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.
must的反意疑问句
如果陈述部分含有表推测的情态动词must,其反问部分不能用情态动词,
应根据具体情况而定.
A．must表示“应该”,其疑问部分用mustn't（不应该）,如：
You must work hard next term,mustn't you
下学期你应该努力学习,对吗?
B．must表示“必须”,其疑问部分用needn't（不必）,如：
We must go at once,needn't we?
我们必须立刻走,是吗?
C．must表示推测,其疑问部分必须与must 后面的主要动词相呼应.如：
1）对现在动作或存在的情况的推测：
You must know the answer to the exercise,don't you?
你一定知道这项练习的答案,是不是?
That must be your bed,isn't it?那一定是你的床,是吗?
2）对过去发生的动作或存在的情况的推测：
①表示肯定
You must have left your bag in the theatre,haven't you?
你一定是把包落在剧场了,是不是?
Aunt Liu must have got to the U.S.A.yesterday,didn't she?
刘大婶昨天准是到了美国了,对不?
②表示否定
表示推测时,否定式通常不是must not,而是can't (cannot).如：
He can't have been to your home; he doesn't know your address,does he?
他不可能去过你家；他不知道你的地址,是不是?
D．陈述部分含情态动词mustn't,表示禁止时,附加疑问部分就可以用must或may,如：
We mustn't be late,must we?(may we?)
我们不可以迟到,是吗?
need
带情态动词need的反意疑问句,疑问部分常用 need +主语.
We need not do it again,need we
当 need 为实义动词时,疑问部分用助动词do + 主语.
She doesn't need to go home alone,does she?