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  1. #1
    Kendama is offline Newbie
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    Default couldn't have VS mustn't have

    I always have trouble teaching the difference between these two to show degrees of negative certainty.

    For example, is there much difference between these:

    He mustn't have been very hungry.

    He couldn't have been very hungry.

    Thanks.

  2. #2
    Tdol is offline Editor, UsingEnglish.com
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    Default Re: couldn't have VS mustn't have

    I'd use the second to contradict or emphasise.

  3. #3
    Raymott's Avatar
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    Default Re: couldn't have VS mustn't have

    Quote Originally Posted by Kendama View Post
    I always have trouble teaching the difference between these two to show degrees of negative certainty.

    For example, is there much difference between these:

    He mustn't have been very hungry.

    He couldn't have been very hungry.

    Thanks.
    I think meaning is more important than trying to rank modals in order of possibility/certainty, etc.

    These sentences can mean completely different things.
    He mustn't have been hungry - because he didn't eat what we gave him.
    He couldn't have been hungry; he had just eaten a seven course meal.


    As far as ranking modals, I believe there are are three basic levels of probability.
    No possibility at all: He won't come.
    A possibility: He [might, may, could] come.
    Certainty: He will come.

    Any other distinctions need to be done with adverbs.

  4. #4
    mmasny is offline Key Member
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    Default Re: couldn't have VS mustn't have

    Quote Originally Posted by Raymott View Post
    He mustn't have been hungry - because he didn't eat what we gave him.
    He couldn't have been hungry; he had just eaten a seven course meal.
    Can't the second one mean that he didn't eat what you gave him too?

  5. #5
    mara_ce's Avatar
    mara_ce is offline VIP Member
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    Default Re: couldn't have VS mustn't have

    Quote Originally Posted by Kendama View Post
    He mustn't have been very hungry.

    He couldn't have been very hungry.
    Iím not a native speaker but I usually teach this type of things.
    In my opinion, the first one doesnít make sense. Iíd say:

    He canít have been very hungry. (couldnít have is possible instead of canít have)

    Must have and canít have are used to make deductions about past actions.

    I must have left my wallet in the car. (Iím sure I did)
    Jim canít have noticed you. (Iím sure he didnít)

    To express possibility or uncertainty about past actions you can use might have, may have and could have.

    Jean might have missed the train. (Perhaps she did)
    He may not have received the letter. (Perhaps he didnít)
    You could have been killed! (It was a possibility)

  6. #6
    billmcd is offline Key Member
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    Default Re: couldn't have VS mustn't have

    Yes.

  7. #7
    Raymott's Avatar
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    Default Re: couldn't have VS mustn't have

    Quote Originally Posted by mmasny View Post
    Can't the second one mean that he didn't eat what you gave him too?
    Yes, it's ambiguous; and that is partly why I say that they shouldn't be seen as merely differences in degree, and why you need adverbials to discriminate meanings.

  8. #8
    Raymott's Avatar
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    Default Re: couldn't have VS mustn't have

    Quote Originally Posted by mara_ce View Post
    Iím not a native speaker but I usually teach this type of things.
    In my opinion, the first one doesnít make sense.
    It does make sense to a native speaker. It means the same as "He can't have been very hungry."

    Must have and canít have are used to make deductions about past actions and states.
    That's right. And since he didn't eat the food we gave him, we deduce that "He mustn't have been very hungry" or that "He can't have been very hungry".
    R.

  9. #9
    mara_ce's Avatar
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    Default Re: couldn't have VS mustn't have

    Quote Originally Posted by Raymott View Post
    R.
    Thank you, Iíve learned something new! I had never seen 'mustnít' associated with certainty. My grammar books relate it to prohibition.

  10. #10
    Raymott's Avatar
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    Default Re: couldn't have VS mustn't have

    Quote Originally Posted by mara_ce View Post
    Thank you, I’ve learned something new! I had never seen 'mustn’t' associated with certainty. My grammar books relate it to prohibition.
    Yes that's true, but you can't prohibit something that has already either happened or not happened in the past. That frees up mustn't for a different meaning in the past. "Must have" is used more commonly than "mustn't have" for the past.

    I'm sure you've heard of this situation:
    Peter: Where is Mary? She was supposed to be here an hour ago.
    John: She must have missed her bus.


    This doesn't mean "She had to miss her bus". It means "I deduce that she has missed her bus."
    Similar principles apply to "mustn't" in the past.

    Grammar books rarely cover the modal verbs comprehensively because there is just too much you could write about them.

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