1

It must be her car, isn't it?

It must be her car, right?

You must be tired, aren't you?

Why it can't be expressed:

It must be her car, mustn't it?

You must be tired, mustn't you?

Note:

Q:Is it her car?

A:Yes, it must be her car.

Possible tag question: It must be her car, isn't it?

Is it possible to ask: Must it be her car? (I've never used it though).

Do native speakers really avoid using tag question of "must"?

3
  • 1
    Wait, why can't mustn't it be used? Commented Mar 19, 2016 at 6:53
  • That's what I've usually heard @DamkerngT. Also, You must be tired, aren't you? So, how to determine?
    – Student
    Commented Mar 19, 2016 at 7:00
  • 1
    Are you learning American English or British English?
    – user230
    Commented Mar 19, 2016 at 7:56

3 Answers 3

1

It must be her car, isn't it?

After a careful investigation I found the following. "The sentence is correct because must is used with tbe idea of deduction." I found this explanation which can tell you why it is used like this. It was also mentioned that such structures are popular in Chinese tests. I think it should be mustn't it? Though I believe the question is rhetorical. People say there's no need to ask for confirmation, because you are sure, so you can just say It must be her car. Or It must be her car, right?

I must do my homework , mustn't I?

Must is used to express obligation.

We can also use don't you think? when asking someone's opinion. Other variants are met in informal English yes,no,right.

It must be her car, right? It must be her car, yes? It must be her car, don't you think?

2
  • Q:Is it her car? A:Yes, it must be her car. Possible tag question: It must be her car, isn't it? Is it possible to ask: Must it be her car? (I've never used it though).
    – Student
    Commented Mar 20, 2016 at 2:31
  • Is tnis her car?.is the most appropriate way.
    – V.V.
    Commented Mar 20, 2016 at 3:11
1

According to [Swan, 1986]:

  1. If the main clause has an auxiliary verb (including a 'modal' auxiliary verb like can, must), the question tag has the same auxiliary.

  2. If the main clause has be, this is also used in the question tag.

  3. If the main clause does not have an auxiliary verb (or be), do is used in the question-tag.

Your example matches 1, so It must be her car, mustn't it? is correct.

2
  • No, the OP's example matches 1, not 2. It must be your car, mustn't it? is what I would say.
    – Colin Fine
    Commented Jan 21, 2020 at 16:56
  • @ColinFine You're right; updated my answer. Swan's 3rd edition is bit clearer about be: If the main sentence has an auxiliary verb (or non-auxiliary be), this is repeated in the question tag.
    – NZD
    Commented Jan 21, 2020 at 20:51
0

I'll make it too obvious for you.

when you use "must" for obligation, the tag must be "mustn't/needn't"

when you use "must" for deduction , the tag can be "mustn't"

OR we can rely on the main verb as if it were without a modal.

NOTICE:

1- you must study hard, mustn't you? [obligation]

2- you must be the nicest one here, mustn't/aren't you? [deduction]

3- you must have played a lot, mustn't/didn't you ? [deduction]

That is the most beneficial short answer you will ever get.

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