can't / mustn't

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sunnyweather

Senior Member
Polish
Hello,

I wonder if both verbs can be used in the following sentences. The key accepts only 'mustn't' in the first and 'can't' in the second, but it seems to me that both verbs are fine in both sentences. Do you agree?

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

You can't use my computer now because I need it.
 
  • Retired-teacher

    Senior Member
    British English
    In the first "can't" is wrong because they are able to speak like that to their parents even though they should not do so.

    "Can't" is correct in the second because its meaning is "unable". "You are unable to use my computer now because I need it".
     

    sunnyweather

    Senior Member
    Polish
    How about: You mustn't use my computer now because I need it.

    I thought it would be OK to use it here. Can't 'mustn't' and 'can't be used as synonyms in some situations? E.g.

    You mustn't smoke here. = You can't smoke here. Isn't it the same?

    I'll be grateful for more clues.
     

    Retired-teacher

    Senior Member
    British English
    "Must not" is close to "should not". "You should not use my computer" is not as strong as the command "You can't use my computer".

    "You mustn't smoke here" and "you can't smoke here" are similar because the person is actually able to do so even though they should not.

    Edit. "You must not use my computer" is forbidding you to do so. "You can't use my computer" is saying that you are unable to do so. I suppose in the context of "because I need it now" they are very similar in effect.
     
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    Enquiring Mind

    Senior Member
    English - the Queen's
    I wouldn't always interpret the difference as strictly as Retired-teacher in conversational usage and in context, sunnyweather. I would say that broadly speaking, you are right. I wouldn't have a problem with "can't" in your sentence 1. The sense is "not allowed to", "don't have the right to".

    "Can (Longman)
    3 ALLOWED
    to be allowed to do something or to have the right or power to do something

    You can’t park here – it’s a no-parking zone."

    So: "you can't speak to your parents like that ...." Lots of examples of "you can't speak to me like that" on Google in the sense "you have no right to ...".

    Even in sentence 2, if "I need it" means "I am going to need it soon", which is perfectly possible, "mustn't" is acceptable, though "can't" would be preferred. If the person who needs it is not actually using it at that moment, the other person physically can, but is not allowed to.
     
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