I heard this sentence in an American film a while ago as I was watching it on DVD (the part after but is verbatim):

"I'm doing my best but I mustn't be doing it right."

This is something I occasionally hear in American films: phrases like "he mustn't have done it" or "she mustn't be studying now", where a logical conclusion is expressed. So far I've thought that the normal thing to say is "he can't have done it" or "she can't be studying now".

My question is: Can mustn't be used to express a logical conclusion when the speaker is certain that something didn't happen or isn't happening, at least in informal speech? Is this an American usage? (I've never heard this usage in British English, but this doesn't mean it doesn't exist.) Is there a change going on in the usage of the particular modal verb?

Note: All the references I've checked don't even mention this use of mustn't. Google books aren't of help either.

EDIT: I should clarify that I'm asking this question because if I wanted to express that I'm certain I'm not doing something right (as in the sentence quoted from the film) I'd say "I'm doing my best but I can't be doing it right". I would think that the use of mustn't/must not wouldn't be standard usage (although the meaning is perfectly clear to me; I never mistook it for an injuction).

  • I would avoid it - it sounds rather dated and "upper class" to me. Which is odd really, because on average, contractions are usually more associated with informal/lower class speech. But In this case I think the norm would be "I'm doing my best but I must not be doing it right." If nothing else, it avoids confusion with the more common "I mustn't" meaning "I really should not". Feb 28, 2012 at 18:56
  • @FumbleFingers: So it's just the contracted form that sounds off to you? Wouldn't "I'm doing my best but I can't be doing it right" sound more normal?
    – Irene
    Feb 28, 2012 at 18:59
  • Yes, you can use contracted can't there (and in fact, using can not would sound stilted). It's just that if you want to use must, you mustn't contract it there (because it'll sound like an injunction rather than a logical conclusion! :) Feb 28, 2012 at 19:12
  • 5
    That's interesting that you wonder if it is American. "mustn't" sounds British to my AmE ears.
    – Mitch
    Feb 28, 2012 at 19:20

4 Answers 4


Per my comment, I'd avoid it because it sounds rather dated and "upper class" to me.

Semantically, the reason for avoiding this construction is simply that it takes the focus off the critical word not. Since the "conclusion" clause is intended to convey something along the lines of "I am failing", this negating word is vital to the sense.

In "injunctive" forms, such as "You must/mustn't do that!", the word must is invariably stressed, to emphasise the intended meaning. In OP's usage, the word must wouldn't normally be stressed, because there's no sense of injunction or stricture (except loosely, in the sense that there's a logically enforced conclusion). It's "not doing it right" that counts, which requires not to be vocalised.

To confirm this particular contraction is nonstandard, note just 3 instances of "I mustn't be doing" in Google Books, but 3830 for "I must not be doing" (almost all for the sense relevant here).

  • Your references from Google Books are convincing. I'm wondering why on earth grammar books don't mention this usage of must not. There must be a change going on which hasn't found its way into descriptive usage yet.
    – Irene
    Feb 28, 2012 at 20:05
  • @Irene: Well, you yourself just wrote "There must be a change going on..." - quite possibly without even noticing. If you can say something must be so, it follows that something incompatible with that must not be so. Grammar books don't normally teach basic vocabulary - nor do many of them specifically point out "plausible" contractions which are "iffy" in some contexts. They don't usually even teach you not to say "I think this to be a very fine point" (although imho that one's really subtle! :) Feb 28, 2012 at 20:50

I personally find the use of the contraction "mustn't" to be a bit off-putting in this case, but I certainly would have no problem expressing an argument this way:

If John had stolen the money, he would have gotten ink on his fingers when the dye-pack exploded.

John doesn't have ink on his fingers.

Therefore, John must not have stolen the money.

Substituting "can't" for "must not" is valid here, but wouldn't be my natural formulation.

(Generally in this situation, the word not has the same stress as the word must, which is why using a contraction there would sound strange to me.)

The same holds true for situations like this:

"It's supposed to work when I do this!"

"You must not be doing it right".

In that case, saying "You can't be doing it right" would sound very unnatural to me. Or, at the very least, like a Briticism.

I tend to think of "mustn't" as a single lexical unit meaning "must avoid", so if someone said "You mustn't be doing it right", I would first think that they were trying to prevent me from doing it the "right" way.

  • Good answer. Personally, I think mustn't sounds really wrong for your first example, but not in your second example. However, I don't know what the difference is. Feb 28, 2012 at 19:44
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    There are a variety of bizarre rules in spoken and written English regarding when you can use the various different forms of clitics. Many of the rules don't make much sense. You can say "He's happy but I'm not" but you can't say "He's not as happy as I'm" instead of "He's not as happy as I am". You can say, "I'm gonna go to the store" but you can't say "You're gonna which store?" instead of "You're going to which store?" Feb 28, 2012 at 21:18

Several replies here address whether it is appropriate to use the contraction "mustn't" rather than writing out "must not" in such an example. Is this your question, or are you asking about the definition of the word "must"? Assuming the latter ...

"Must" can mean "obligated, required", as in, "You must pay your taxes by April 15", or "You must be quiet while the teacher speaks." That definition doesn't make sense in this context. But "must" can also mean "compelled by necessity, inevitable", as in, "If you hold a rock in the air and release it, it must fall to the ground". The rock is not compelled to fall by the laws of the state legislature, but by the laws of physics.

That's the sense in which "must" is used in contexts like your example. "If A is true, then B must be true" means that B follows inevitably from A because of laws of nature or mathematics or logic. "If the engine is getting fuel, then the problem must be in the electrical system." "Must" here doesn't mean that the problem had better start being in the electrical system or we will subject it to public ridicule or have it arrested. It means that it is (or at least, the writer believes it to be) a logical inevitability.

  • Nice explanation of the verb must. What I asked is whether the negative form (whether contracted or not) can be used to talk about a logical conclusion, i.e. use it to say I'm certain something happened although I don't know for a fact. I know I can say "This can't have happened" when I am certain it hasn't happened. Can I say "This mustn't/must not have happened" and mean the same thing?
    – Irene
    Feb 28, 2012 at 22:11
  • ... Apparently, I can. I was wondering though if this is standard usage (since I can't find reference for this usage of the verb must not).
    – Irene
    Feb 28, 2012 at 22:13
  • Short answer: Yes. Sorry, I guess I should have been more explicit. But if you can say "this must be", you can logically also say "this must not be". I'm not prepared to say that it is always true that "not X" is grammatically and logically valid in any context where "X" would be, but that is the normal case.
    – Jay
    Feb 29, 2012 at 14:41

When considered deductions, can’t be or can’t/couldn’t have been is negative of must be.

Mustn't be is also negative of must be meaning that there is no possible way that it could be otherwise. It implies the certainty of the speaker's own claim.

Please compare:

a) ... but I must be doing it wrong.
b) ... but I can't be doing it right.

a) ... but I must be doing it wrong.
b) ... but I must't be doing it right.

Edit 1

Irene, with reference to your sentence

... I'd say "I'm doing my best but I can't be doing it right".

To construct the sentence in this way, the sentence in question should have been "I'm doing my best but I must be doing it wrong.

Think about how the words "right" and "wrong" contribute to "deduction" together with "must be", "mustn't be", "can't be".

  • I think that your examples contradict what you say about can't being the opposite of must.
    – Irene
    Feb 28, 2012 at 20:27
  • @Irene, please see my edit.
    – Mustafa
    Feb 28, 2012 at 21:09
  • must't? did you mean 'can't'?
    – Line
    Mar 24, 2021 at 9:54

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