I have been told that 'unless' can't be used in hypothetical clauses as follows:
Unless Ann had studied hard, she would have failed the exam.
My question is, why is using 'unless' in the conditional clause above unacceptable?
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Because we think of a hypothetical situation as not happening yet, or hard to happen and for this reason, no one can be sure whether the situation can happen or not, "unless" doesn't feel appropriate to use for describing the hypothetical situation.