Re: That boy __________ be Jason
The easiest answer is that 'must' can used to suggest logical certainty of an affirmative situation , and 'can't' for logical certainty of a negative situation.
Originally Posted by keannu
'Must' can alo be used to impose an obligation to do something, 'mustn't' to impose an obligation to not-do something, and 'don't have to/needn't' to state absence of obligation.
'Must not' is rarely used to suggest logical certainty of a negative situation, but it can be, 'Mustn't' is used in this way so rarely, and by so few speakers, that for practical purposes we can say that it is not so used.
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