can't / mustn't speak like that

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Senior Member
I wonder if both options are possible in the examples from English in Mind 3 (CUP 2010)

1) You mustn't / can't speak to your parents like that. It's very disrespectful.

2) You mustn't / can't use my computer now because I need it.

Thank you very much.
  • cyberpedant

    Senior Member
    English USA, Northeast, NYC
    As the course originates in Britain, I'd say "mustn't" is the word you're looking for in the first sentence.
    The second I find ambiguous, but "mustn't" has never been part of my active vocabulary.

    Enquiring Mind

    Senior Member
    English - the Queen's
    I'd say "can't" or "mustn't" are possible in both sentences. Can (and therefore can't) also has the meaning of "to be allowed", e.g.: Can I borrow your pen? Can I smoke in here?


    Sixties Mod
    English UK Southern Standard English
    I would use "mustn't" in the first and "can't" in the second. But they both so often convey the same meaning of not allowed to that I wouldn't be at all surprised to see it done the other way round.
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