Why is the verb were use used in hypothetical questions even with a singular subject? i.e. if she were my .

Answers:
I think this is a question of verb tense, not subject agreement. You know, it's like the future perfect tense of "is"..or something like that.

To say "If she was ..." has an implication of the past, like "If she had been"

Nothing to do with singular v plural. The distinction in this case is between the subjunctive and the indicative tenses.

look it up under conditional modals...