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Dear teachers,
He must be a teacher, mustn't he?(I know this sentence is correct).
He must be a teacher, isn't he?(Is it correct?)
Should we have to use 'must' in question tag if we use 'must' in the previous sentence.
Is there any special situation?

Thank you in advance.^_^
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hey,

when we use must with the idea of obligation we use mustn't to make the tag question

when we use must with the idea of assumption/deduction we use another auxiliary verb do make the tag qusetion such as to be or do/does.

examples:

You have worked a lot. You must be tired, aren't you?

She must be the new teacher, isn't she?

Wow, Robert is a flight attendent, he must fly a lot, doesn't he?

I must leave now, mustn't I? (idea of obligation, in this case the obligation is being imposed on you)
Students:Teacher, we must bring our homework next class, mustn't we?
Teacher: Yes, you must.

In your example He must be a teacher, mustn't he? sounds like He has to be a teacher, doesn't he?

if the idea of the sentence is of assumption I'd say: He must be a teacher, isn't he?
Thank you,rafaelinrio. I understand.^_^
I also have a little problem with answering the Tag question.

You have worked a lot. You must be tired, aren't you?
Yes, i'm very tired.(agree with the speaker)
No, i don't tired.(disgree with the speaker)

She must be the new teacher, isn't she?
yes, she is.(agree with the speaker ,means she is a new teacher)
No, she isn't.

No one is perfect, aren't we?
yes, we are( means we are not perfect)
No, we aren't.(means we are perfect)
Am I right?
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I can't really agree with this. Although I don't think I've ever heard an American say "mustn't he" you really need the verbs to correspond. If you wanted a tag, I'd be more likely to say "He must be a teacher, right?" or "I bet you're tired, aren't you." I know there's been a thread on this already - I'll see if I can find it.
Well, there are a few threads out there. There's this one, for example: http://www.EnglishForward.com/English/QuestionMust/zlkgr/post.htm Anyway, I wouln't mix the "must" and "to be."
Thank you very much G G. ^_^
I won't use must in Tag question too. But there always have exams. ^_^
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You have worked a lot. You must be tired, aren't you?
Yes, i'm very tired.(agree with the speaker)
No, i don't tired.(disgree with the speaker)

In this case as verb to be works as the auxiliary verb of the tag question, you've got to use it in the answer, then you have 2 possible answers:
Yes, I am.
No, I am not.

Concerning the other examples you gave,

No one is perfect, aren't we?
yes, we are( means we are not perfect)
No, we aren't.(means we are perfect)
Am I right?

The first case is incorrect because No one is a negative word and requires a verb in the affirmative,so we have:

No one is perfect, are they?

Yes, they are. (they are perfect)

No, they aren't (they aren't perfect).
Yeah, but someone told me that 90% of question tags are Emotion: stick out tongueositive verb-negtive question tag verb or negtive verb-positive question tag verb. and another 10% of tag question may have the same polarity.( not normal)
Eg:
I look through the window and say:'It's going to rain, is it?' ( The speaker expect you to take look at outside and agree or disagree)
Hi Lynn,

Don't confuse the response with the tag in terms of negative/positive.

It's going to rain, isn't it? (You expect it is going to rain.)
It's not going to rain, is it? (You do NOT expect it to rain.)

The person can respond to either one with either of the following: No, I checked the forecast, and it's not supposed to rain. Yes, looking at the clouds, I'm pretty sure it's going to rain.

Sometimes you do have a positive/positive in the tag itself. This usually expressed agression/a challenge, disbelief, or surprise. These are much less common.
He won first place, did he?
You're going out of the house dressed like that, are you?
You think so, do you?
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