He mustn't / couldn't have been hungry



He couldn't have been hungry.

He mustn't have been hungry.

Is there a difference in meaning between those two?


Posted 2013-06-23T06:52:32.427

Reputation: 7 310



(I think I'm going to open a can of worms with this answer but I've done some research so, don't blame me.)

In the student's text book, New English File Upper-Intermediate Oxford University press Page 138 it says:

The opposite of "must have" is "can't have" NOT "mustn't have"

So for some it is considered standard English to use: can't have or couldn't have instead of mustn't have when you are speculating or guessing about the past in questions and negative sentences.

  • He couldn't have been hungry

    means practically the same as

  • He can't have been hungry

They both express a strong conviction in the past, the speaker can choose to add further information in order to back up his claim.

A: John didn't eat his cereal this morning.

B: He can't/couldn't have been hungry. He usually has breakfast.

Thus the speaker is saying it's impossible that John was hungry because he knows John never leaves home without eating something. Must not (mustn't) means something quite different, you are forbidding someone or something from performing an action now, in the present and it is not used for speculating in the past.

On p394 in Practical English Usage by Michael Swan:

Must is used with the perfect infinitive for deductions about the past.

  • "The lights have gone out" -- "A fuse must have blown."
  • "We went to Majorca." -- "That must have been nice."

Must is only used in this way in affirmative sentences. In questions and negatives, we use can and can't instead.

This is also confirmed by A Practical English Grammar by A.J.Thomson A.V. Martinet 4th edition on page 148.

Mari-Lou A

Posted 2013-06-23T06:52:32.427

Reputation: 19 962

3Two recent grammars, the descriptive Cambridge Grammar of the English Language (2002) and the pedagogic Cambridge Grammar of English (2006) note that mustn't is used to express negative deductions. Their examples include: You mustn't have filled it in. He mustn't have done it deliberately. He mustn't have told her after all. The latter grammar notes that such a use of mustn't is especially found in informal spoken contexts. – Shoe – 2013-06-23T10:50:59.320

Thanks, I'm surprised I haven't received any negative feedback so far. Oh, and by the way I also consulted two English/Italian grammar books which confirmed the standard/formal rule. – Mari-Lou A – 2013-06-23T11:09:55.110

Although this use of "mustn't have" is rare, it is indeed used as a synonum of "can't have", "couldn't have" in this context. (Google Ngrams shows that this has become more common recently in American English, so for British or Commonwealth English, this answer is good grammatical advice. Without Ngrams, I never would have guessed this was mainly an Americanism.) – Peter Shor – 2013-06-23T16:01:40.097

2@PeterShor As an American, I must say that mustn't have sounds like British English from a hundred years ago to me ;) I've never heard must used this way. Sentences like Mari-Lou's two must have examples sounds fine to my ear, but mustn't have? I think to myself "...couldn't been allowed to have had? What does that even mean?" So Ngrams might be throwing a wrench in the works in this case ;) – WendiKidd – 2013-06-23T16:09:17.907


@WendiKidd: Here's the Ngram. In 1800, Ngrams says that American and British usage of "must not have been" was similar, but American usage seems to have increased substantially since then. Upon reflection, I do have the impression that the contraction of "must not" to "mustn't" is more common in the UK. Google Ngrams expands "mustn't" to "must not".

– Peter Shor – 2013-06-23T16:17:47.933

@PeterShor The fact that "must not" can and is often contracted to mustn't is immaterial to whether the meaning is the same as "can't" and "couldn't" when speculating about the past. There is no real justification nor need to adopt a third modal construction; however, the OP has either been taught or has learnt this newish formula. Now that is relevant and interesting. – Mari-Lou A – 2013-06-23T18:05:10.063

+1 But Mary must have some problem does not exhibit a 'past infinitive', and it represents a deduction about the present, not the past; have there is a lexical verb, not an auxiliary. You want *Mary must have had* some problem.* (This is not to say that Mary must have some problem is unacceptable, just that Swann's dictum doesn't apply.) – StoneyB on hiatus – 2013-06-23T19:17:55.593

Oops! I had included that example in a previous draft and left it there without thinking. Thank you, I'll make amends. – Mari-Lou A – 2013-06-23T19:21:13.713

Then please make clear that must is used, without regard to tense, to represent deductions as opposed to impossibilities. – StoneyB on hiatus – 2013-06-23T19:24:19.917

3@PeterShor I agree that must not is rarely contracted in US speech. Must not have is usually contracted to *must not've*. But the whole Ngram thing is complicated by the fact that deontic must has virtually disappeared from US speech. – StoneyB on hiatus – 2013-06-23T19:26:51.100

1I agree with WendiKidd. To my AmE ear, *"he mustn't have been hungry"* expresses a strong prohibition, while *"he must not have been hungry"* expresses a strong statement of probability. The former, to me, is nonsense. From this point of view, it's probably simplest to consider mustn't a separate word rather than a contraction of must not. (And so, it's probably best not to use the Google books corpus for this particular question.) – snailplane – 2013-06-24T08:48:06.613

@snailboat that's curious. So the mere fact that the negative form is contracted changes its meaning. Which reminds me of the paradoxical meaning of don't have to compared to have to. Am I right? – Mari-Lou A – 2013-06-24T08:53:34.137

@Mari-LouA I think it does, which is why I suggested considering it a separate word rather than a contraction. (Obviously it is a contraction etymologically, but so is goodbye.) I do hope it's clear, though, that I was expressing my personal observations in my comment. I haven't done enough research to say what's true in general for other speakers. – snailplane – 2013-06-24T09:08:28.120


I think 'mustn't' could be dialectical - ie, more common in Irish English and Scottish English. And in contrast to what WendiKidd says, to me it sounds more of a strong probability in the contracted form, rather than more of a certain deduction, ie, he must not have been hungry, ie, he certainly must have been unhungry (I know that doesn't exist).


Posted 2013-06-23T06:52:32.427

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