'Unless' in hypothetical constructions

Solution 1:

You're correct, that sentence

• ?Unless Ann had studied hard, she would have failed the exam.

is not fully grammatical. OK, that happens. What makes it interesting is that, normally, unless means if not. And if you use if ... not instead of unless, everything works just fine:

• If Ann had not studied hard, she would have failed the exam.

But it clearly doesn't work with unless very well. The effect of this sentence on a native speaker is telling. The meaning is reasonable but the parsing sends up a flag. Frankly, I don't know why. It must be something about putting the negation and the if together in one lexical item, but why it's flagged here is very puzzling.