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Why it is acceptable to use (B) to make a deduction (about the past) and not (A) (about the present)?

A- Mustn't + base verb ( about the present) Example: (1) The answer mustn't be A.

B- Mustn't + have + past participle ( about the past) Example: (2) He mustn't have expected that to happen.

Is it a semantically matter? Because it's true, it feels odd to say (1)

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    I've heard that it's more American than British to say B. The British people prefer to say "CAN'T have + Past participle". Is That right? Oct 17, 2022 at 13:31
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    I'm British, and I am quite happy to say, e.g. 'He mustn't have looked, and that's why he was killed by the train'. Oct 17, 2022 at 14:08
  • There is no rule-based reason for why (1) is in incorrect while (2) is correct. If you're learning English, you just have to learn the different functions of the various modal verbs. The reason for the difference is a historical one, and questions about etymology are best left to our sister site, english.stackexchange.com.
    – gotube
    Oct 17, 2022 at 14:46
  • Scenario: Writer 1: Let's finish this dialogue and the questions that go under it. Writer 2: Yes, let's do. Except the answer mustn't be A. We should make it B.
    – Lambie
    Oct 17, 2022 at 15:01
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    @MichaelHarvey I too am British and agree with you.
    – WS2
    Aug 7, 2023 at 17:09

2 Answers 2

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I am British and am perfectly happy with B

The problem in using A is that it is ambiguous, and that is clearly why we do not use it in the way you suggest.

"The answer mustn't be A" is used in the sense of "Work out how much you will need for the project, but the answer mustn't be more than £10,000 (otherwise it will be rejected".

I believe you are trying to make it mean "There is someone knocking at the door but it mustn't be John, because he always rings the bell". That does not work. You would have to say something like "...it can't be John because...".

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For context, I'm a native speaker of American English. Everything that I say here is in the context of American English, specifically. I can't speak to the nuances of other varieties or dialects of English.

"Must not" (present tense) and "must not have" (past tense) can both be used to for deductions or inferences. For example, the following sentences sound fine to me:

(1) "I applied to work at that company, but they never contacted me for an interview. They must not have thought I was a good fit for the position."

(2) "My teacher must not have noticed that typo in my essay, because I got a perfect score on it."

(3) "I have left several suggestions on their feedback forms over the years, but they have never fixed anything that I complained about. They must not take those forms very seriously."

(4) "It looks completely dry outside. It must not have rained last night, like the weather forecast said it would."

When used for deductions, "must not (have)" is roughtly the same as "mustn't (have)." However, "mustn't" is more likely to be used for prohibition than for deduction, while "must not" is more likely to be used for deduction than for prohibition. For example:

(5) "We must not be late." (Deduction: "I'm assuming that we aren't late.")

(6) "We mustn't be late." (Prohibition: "It's important that we not be late.")

When used to express prohibition, "must not"/"mustn't" often mean the same thing:

(7) "You must not tell anyone about this." (Prohibition: "You are not allowed to tell anyone about this.")

(8) "You mustn't tell anyone about this." (Prohibition: "You are not allowed to tell anyone about this.")

However, there is no past tense for the prohibition sense of "must not" and "mustn't." They only exist in the present tense. "Must not have" and "mustn't have" always mean deduction:

(9) "They must not have seen us." (Deduction: "I think that we went unnoticed.")

(10) "They mustn't have seen us." (Deduction: "I think that we went unnoticed.")

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