I'm wondering "must not be" or "must not have been" should be used in the following:

This defamatory letter must not have been written by John / must not be written by John. He's the kindest guy I've known.

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    "The letter cannot have been written by John" would be more natural. Commented Mar 13, 2021 at 12:56
  • Thank you. Is it correct to say "The letter cannot be written by John"i n this scenario?
    – Apollyon
    Commented Mar 13, 2021 at 13:17
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    It's possible, but I think the past tense is more idiomatic. The sense is "It isn't possible that John wrote this letter." Commented Mar 13, 2021 at 14:01
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    Both versions are fine, and in any but the most contrived contexts, mean exactly the same thing. Just as John couldn't have done it and John can't have done it, for example, are usually equivalent and interchangeable. Commented Mar 13, 2021 at 14:02
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    Does this answer your question? "can't" versus "couldn't", what is the difference? But see also How should I use 'must' in past tense? and similar questions, because "must" is a funny verb when it comes to the past tense. Commented Mar 13, 2021 at 14:04

1 Answer 1


I say that “must not be” should be used in the sentence.

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