1、在回答must開頭的一般疑問句時,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開頭的一般疑問句時,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開頭的一般疑問句時,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的反意疑問句
如果陳述部分含有表推測的情態(tài)動詞must,其反問部分不能用情態(tài)動詞,
應根據具體情況而定.
A．must表示“應該”,其疑問部分用mustn't（不應該）,如：
You must work hard next term,mustn't you
下學期你應該努力學習,對嗎?
B．must表示“必須”,其疑問部分用needn't（不必）,如：
We must go at once,needn't we?
我們必須立刻走,是嗎?
C．must表示推測,其疑問部分必須與must 后面的主要動詞相呼應.如：
1）對現在動作或存在的情況的推測：
You must know the answer to the exercise,don't you?
你一定知道這項練習的答案,是不是?
That must be your bed,isn't it?那一定是你的床,是嗎?
2）對過去發(fā)生的動作或存在的情況的推測：
①表示肯定
You must have left your bag in the theatre,haven't you?
你一定是把包落在劇場了,是不是?
Aunt Liu must have got to the U.S.A.yesterday,didn't she?
劉大嬸昨天準是到了美國了,對不?
②表示否定
表示推測時,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?
他不可能去過你家；他不知道你的地址,是不是?
D．陳述部分含情態(tài)動詞mustn't,表示禁止時,附加疑問部分就可以用must或may,如：
We mustn't be late,must we?(may we?)
我們不可以遲到,是嗎?
need
帶情態(tài)動詞need的反意疑問句,疑問部分常用 need +主語.
We need not do it again,need we
當 need 為實義動詞時,疑問部分用助動詞do + 主語.
She doesn't need to go home alone,does she?