6

He couldn't have been hungry.

He mustn't have been hungry.

Is there a difference in meaning between those two?

7

(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.

  • 3
    Two 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 Jun 23 '13 at 10:50
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    @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 Jun 23 '13 at 16:09
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    @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 Jun 23 '13 at 16:17
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    @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 Jun 23 '13 at 19:26
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    I 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.) – snailboat Jun 24 '13 at 8:48
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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).

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