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Senior Member
In "SpeakOut Pre-Intermediate" (by Antonia Clare and J.J. Wilson) there's a grammar exercise in which you need to complete the sentences with "can/can't/have to/must/mustn't". It's about the exam rules. I'm curious about the second part of the following sentence:

"You have to arrive on time, or you .......... come into the examination room."

The key says the answer should be "can't". And I agree. But what about "mustn't"? Why is it incorrect?
  • The Newt

    Senior Member
    English - US
    "Can't" means that you will be prevented from entering; "mustn't" simply says (quite forcefully) that we don't wish you to enter.
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