It ___ be Thai or Indonesian, I think! (must / can't /mustn't)

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ingarten

Senior Member
Ukrainian - Russian
Hello!
There's a test question that bothers me. There's a gap to be filled.

A What kind of food is this?
B I'm not sure. It ___ be Thai or Indonesian, I think!
A must B can't C mustn't

I know "mustn't" is wrong as it expresses prohibition. So, I really need to choose between A must B can't.
Could you explain why option A is way more better than option B? There must be grammar rules that explain this case.

Thank you very much!

P.S.: It is known that 'must' can imply strong probability which means that the speaker thinks that it is probably Thai or Indonesian food. 'can't' vice versa shows that it's highly unlikely that the food is Thai or Indonesian that make both variants grammatically correct

Is taken from: English File Intermediate (Oxford)
 
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  • PaulQ

    Senior Member
    UK
    English - England
    The question is not very good - you should ignore it.

    The "I think!" is not idiomatic at all1 - what is required is a tag question.
    B1 I'm not sure. It must be Thai or Indonesian, mustn't it?
    B2 I'm not sure. It can't be Thai or Indonesian, I think can it!?

    B1 I'm not sure. It must be Thai or Indonesian, I think! -> This is wrong because "must" expresses certainty, but "I think expresses doubt. You can correct it by saying
    "I'm not sure. I think it must be Thai or Indonesian,
    B2 I'm not sure. It can't be Thai or Indonesian, I think can it?

    1 Edit - I have just thought that "B1 I'm not sure. It must/can't be Thai or Indonesian... I think!" could work, in which the speaker corrects his earlier certainty.
     

    grassy

    Senior Member
    Polish
    Both are grammatically correct but I think A is the more likely answer. The problem with B is the following: what would make one specify that it's neither Thai nor Indonesian? If it's not Thai or Indonesian, it's probably not German or Slovak either. What's the point of mentioning what it is not? We'd seem more likely to guess what it is.
     
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    Enquiring Mind

    Senior Member
    English - the Queen's
    Hi ingarten, I would say the correct option is dictated by the affirmative "I think" at the end, which means the main verb must be in the affirmative, so it must be A.
    It must be Thai or Indonesian, I think! :tick:
    It can't be Thai or Indonesian, I think! :cross:
    It can't be Thai or Indonesian, in my opinion. :tick:
    It can't be Thai or Indonesian, if you ask me. :tick:
    It can't be Thai or Indonesian, I don't think! :tick:
    It mustn't be Thai or Indonesian, I think! :cross:

    "Mustn't" is problematic here, particularly in BrE; see Note: MUST with a negative - three-quarters of the way down this page (usefulenglish.ru):
    Negative constructions with MUST in the meaning "strong probability" are not used very often. Their use is complicated, and their meaning is not always clear. It is better for language learners to use substitutes (e.g., probably) instead of MUST with a negative in the meaning "strong probability". (...) MUST NOT in the meaning "strong probability" is used mostly in American English, though not very widely. (...) In such cases in British English, "can't" is usually used instead of "must not".
     
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