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Which choice should be used here, and why?

You ought to do your homework, oughtn’t/shouldn’t/mustn’t you?

My book says mustn’t, but that’s not the same verb in the tag question as in the sentence proper, so this is confusing.

3

As a warning to English language learners who read this: the only sentence that sounds completely like modern, everyday speech to me is this one:

You should do your homework, shouldn't you?

However, given the constraint that we can't change "ought" in the first part of the sentence, the only correct answer here is "oughtn't you."

This expression in English is a form of ellipsis:

the omission of one or more words that are obviously understood but that must be supplied to make a construction grammatically complete

The auxiliary alone is used to imply the entire idea. For example, in the following, much simpler example, the ellipsis is shown in parentheses:

You can do it, can't you (do it)?

Because the second auxiliary verb is being used as a reference to the the entire idea introduced by the first auxiliary verb, they must always match.

This isn't as obvious in the present and past tenses, but is still true, because there is always an implied auxiliary "do" in these forms, which is used to create questions and negations:

You went, didn't you (go)?

Using "ought" in the first part of the sentence complicates this, because the somewhat archaic "ought" is used these days with an infinitive, but the nearly obsolete negative question form is still used as a pure auxiliary (notice the to here goes away):

You ought to do your homework, oughtn't you (do your homework)?

I would never use this construction with ought, at least partially because of the fact that almost no one ever says "oughtn't" anymore! Regardless, I don't know how the book could possibly come to the conclusion that "mustn't you" would ever be correct with "ought."

  • nice info, but where is my answer in the above? – kumar Dec 23 '17 at 7:10
  • You edited your question after I answered. You originally asked which is correct, and I said they all are. You are now asking which should be used, and I told you my first example is the most natural. What else do you want answered? The other part you added about your book? If it says to use "mustn't you," I think it is teaching you archaic language. If it says to mix "ought" with "mustn't you," I think it is flat wrong. But maybe people speak English that way, somewhere. – joiedevivre Dec 23 '17 at 7:24
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    All of them sound extremely odd with "ought" in the first part of the sentence, and I would never say any of them. But at least "oughtn't you" doesn't sound downright wrong. It does sound completely wrong to me to use different modals in the two parts! In this construction, the second verb auxiliary always refers to and matches the first one. You will go, won't you? You wouldn't do that, would you? You can do it, can't you? You are lost, aren't you? – joiedevivre Dec 23 '17 at 7:53
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    I just up-voted this answer after first contemplating a down-vote. Because I believe the answer to be thorough and technically correct, why was my first reaction so negative? The reason is that "oughtn't" is so rarely used today that advising a learner to use it seems perverse. The answer should be revised so as to make the final paragraph the introductory paragraph. And actually I do say "mustn't." – Jeff Morrow Dec 23 '17 at 14:42
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    @Joie You are correct that I almost certainly would use "should/shouldn't" in this context. I was reacting to what I believe was fairly broad statement about "mustn't" in an earlier version of your answer. I reiterate that upon consideration I not only agreed with your answer but also voted for it. The original question was unfortunate, and any answer was going to be difficult. – Jeff Morrow Dec 23 '17 at 22:52

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