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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    Unlike if constructions, you can't use it if you know whether or not she failed her exams. But if you don't know, it's fine. – Peter Shor Jul 12 at 18:35
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    There may be some quirky usage rules that apply to unless, but that one isn't one of them. – Phil Sweet Jul 12 at 18:36
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    But you can say "Unless Ann studies hard, she will fail her exam". – Kate Bunting Jul 12 at 18:48
  • I think they are probably confused with not using will or would in the clause following unless (the subordinate one). It is fine in the main clause. See here unless – Phil Sweet Jul 12 at 18:58
  • @Kate Bunting: l know that it's ok to use it in present and past simple conditional clauses. But l was wondering why it's unacceptable in past perfect conditional clauses. – Mido Mido Jul 12 at 21:12

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.

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