Cfrtjryk

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?

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.

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.

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.

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.

A: We must play a game now.

B: Must we?

As I understand 'must' is used to specify very sharp obligation, negation of which is a counter obligation, because it presumes there is no other option. E.g./similarly by saying "This number is not even."
we implicitly mean that "This number is odd." That is both "must" and "must not" are sharp obligations and if you want to express there is not such obligation, you need to drop that obligation (must) altogether.

So when someone says "We must play a game now". You could say:

There is not such a must.

:-)