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Let me explain. Suppose someone says "We must play a game now". I disagree, but only on that we have to do it.

I shouldn't answer "We must not" because I would be saying that the game is not to be allowed to happen.

"We don't must" feels incorrect, I'm not sure if it is.

So, other than completely changing the phrase answering things like "I don't feel like playing" or "I disagree", how do I express that idea that I don't think that we must, using the verb must? Is it possible?

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More about this on wikipedia – krubo Sep 11 '11 at 14:56
up vote 5 down vote accepted

The key issue is the ordering of auxiliaries in English.

(Subject) + Modal + PerfectMarker + ProgressiveMarker + PassiveMarker + (MainVerb) + (...)

An example of all the auxiliary positions being used is:

We must have been being taught English grammar.

Negatives occur after the element they are most relevant to (often the first). Since must is a modal, it occurs in the first slot. But this blocks any other modals from occurring there. Adding not runs into the conundrum of meaning you already mentioned. You could attempt to insert another expression of necessity later in the sentence, but having multiple such words often changes the semantics in odd ways. For example:

We must not have to play a game.

(Italics to indicate that most speakers would emphasize "not") It seems right at first, but this expresses that you believe (or you've come to the conclusion) that we don't have to play. This is not precisely the same thing, and the sentence is a little odd as well.

Also you are right: you cannot generally say "We don't must" as the dummy and emphatic forms of "do" cannot occur with any auxiliaries, and even if they could, they could not occur before must. (The 'lexical' do still can, but this occupies the main verb slot, as in "We must not do that.")

Thus our only real choice is to reword the sentence and lose must. Generally the most native-sounding choice is to use a periphrastic modal like "have to". (Note that periphrastic modals occur in the main verb slot, as they have not fully grammaticalized into real modals. Yet.) I think the choice the other respondents both used sounds the best:

We don't have to play a game.

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A very nice and thorough answer. How about "We must have to not play a game"? :) – Kit Z. Fox Sep 9 '11 at 1:30
I would avoid using both must and have to. I'm not an expert on semantics, so I can't explain very well how they work together, but usually the result is messy. Now I have no evidence or knowledge to back this up but I hypothesize that when you use them both, have to represents the necessity and must is forced to be interpreted as a marker of certainty (see 5b at thefreedictionary.com/must) In any case, I would interpret that sentence as "It seems to me that it is necessary for us to not play," which is close to the original "We must not play". – tdhsmith Sep 9 '11 at 2:37
I don't see how the ordering of auxiliaries are related to this. As far as I can see the main issue is only the conundrum of meaning – Louis Rhys Sep 9 '11 at 7:50
Well I wanted to demonstrate that you couldn't use "must" to achieve the meaning desired because 1) nothing can occur before it to modify it (and things afterward cannot modify it, generally), 2) negating it does not produce the right meaning, and 3) adding another modal forces "must" to have a different, undesired interpretation. In a sense this is just the conundrum of meaning as you say, but I wanted to demonstrate explicitly why there weren't other options. It was probably a little over-the-top, yes. ;) – tdhsmith Sep 10 '11 at 3:04

To get the logical negation of "must", switch must with a different verb.

A: We must study English grammar.
B: No, we need not study English grammar.
or B: No, we don't need to study English grammar.
or B: No, we don't have to study English grammar.
or B: No, we are not required to study English grammar.

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It's always better to answer using the provided question as sample :) – Chibueze Opata Nov 14 '12 at 6:54

Remember that one of the defining characteristics of modals is that they can't function as infinitives. If you bear that in mind, it helps you predict the impossibility of what you are suggesting and similar sentnces.

So, the solution is that you need to find another periphrastic construct (usually "need to" or "have to" etc) with a similar meaning.

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Normally, a native English speaker would say something like:

We don't have to play a game.

They'd probably say it with an accent on "have" to denote that their intention is not the fact that they aren't playing the game, it's just that playing the game isn't an absolute necessary.

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A: We must play a game now.

B: Must we?

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