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In this question, the OP wrote this:

She must leave early, mustn't she?

But I read this sentence here:

You must go, needn't you?

I read about must have here, but it doesn't say explain a lot about must as present modal. I only found this sentence:

He must use the lift, mustn't he?

Which one is correct here? Also, these are confusing me too:

You must be tired, ____?

That guy must be rich, ____?

You must visit me every day, ____?

You don't seem to be a happy couple. You guys must fight a lot, _____?

According to what I can understand, the word must has several forms of tag questions.

I have googled and read some references, but the explanations seem to be overlapping.

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  • Safira, please make it a lot more clear by posting the answer you prefer for each of the examples, preferably with one more answers you think are wrong. Commented Sep 16, 2016 at 22:02
  • You mean I should write my answer on the question itself?
    – Safira
    Commented Sep 19, 2016 at 2:25
  • None of these sound idiomatic. use "Right?" Or similar.
    – Stuart F
    Commented Mar 16 at 4:47
  • @Safira Yes… not in the Question, but in the exposition. Either way, please be sure Stuart F is correct. While 'She must leave early, mustn't she?' might not really be wrong, it will be unusual. 'You must go, needn't you?' is not comparable, except in the sense of good or bad… No-one is likely to misunderstand, but that phrase is largely nonsense; guaranteed to show you up as inexpert in English. Your other examples, more usefully, show why such statements need not two clauses separated by a comma but rather two separate sentences, only the first using 'must'. More… Commented Mar 18 at 0:09
  • Further… Setting aside Stuart F's fine advice, any or all of those examples could as easily be positive or negative… You must be tired. Are(n't) you? He must be rich. Is(n't) he? You must visit me every day. Will/won't you? You don't seem to be a happy couple. You guys must fight a lot. Do(n't) you? Commented Mar 18 at 0:11

3 Answers 3

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Safari, I meant I didn’t really see what your question was and examples of what you consider wrong answers might have helped.

‘You must go, needn't you?’ has the right structure but the words are mismatched.

‘You must go, mustn't you?’ corrects the mismatch.

‘You need to go, needn't you?’ would be wrong because of the different verb form. ‘You need to go, don't you?’ would correct that.

There are obvious endings for your examples but I’m not sure I’ve understood the question.

‘You must be tired, mustn’t you?’

‘That guy must be rich, mustn’t he?’

‘You must visit me every day, mustn’t you?’

‘You guys must fight a lot, mustn’t you?’ is correct, but we might be just as likely to hear ‘You guys must fight a lot, don’t you?’. That's because the ‘must’ seems unnecessary; ‘You guys fight a lot’ would be simpler, and lead us to expect ‘don’t you?’

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  • I perceive a difference depending on whether must has a deontic or epistemic meaning. "You must go, mustn't you" sounds fine, but "That guy must be rich, mustn't he" grates on my ears like fingernails on a chalkboard. Commented Mar 16 at 12:47
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Musn't in a tag question is sometimes fine -

That guy must be rich, musn't he?

... but quite rare.

If a tag question is asking a real question then alternatives to must are often preferred:

She must leave early, mustn't she? / She has to leave early, hasn't she?

He must use the lift, mustn't he? / He has to/needs to use the lift, hasn't he/doesn't he?

Some of the OP usage examples seem more like assertions than actual questions requiring a tag. In these cases a simple affirmative or negative can be used -

You must be tired, no?

You must visit me every day, OK?

... or nothing at all -

You don't seem to be a happy couple. You guys must fight a lot.

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    It seems some English teachers or students are determined to find a tag question for every sentence, but sometimes it's just not idiomatic to use one: it's far more common to say yes/no, OK, right, etc.
    – Stuart F
    Commented Mar 16 at 22:27
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When you are using must for deduction or probability then you can't use mustn't. Substitute with be in the negative form. i.e.

‘You must be tired, mustn’t you?' -> right answer : You must be tired, aren’t you?

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    Commented Mar 16 at 2:01

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