Mustn't have or can't have
I found following question which ask to select a suitable phrase from the given options. The correct answer for this is "can't have", though my answer is "mustn't have worked". The main reason for my answer was if she had work hard, she would not have got bad results. Therefore, please someone tell me why "mustn't have" is inappropriate for this sentence. Furthermore, I would be really grateful if someone explain what is the difference between "mustn't have + past participle" and "can't have past participle".
She got a terrible mark in the exam so she mustn't have worked / can't have worked at all.
Top answer
It just happens than, in BrE at least, we use must (have) in the affirmative and can't have in the negative to express logical deduction.
- It just happens than, in BrE at least, we use must (have) in the affirmative and can't have in the negative to express logical deduction.
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It just happens than, in BrE at least, we use must (have) in the affirmative and can't have in the negative to express logical deduction.
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