This question already has an answer here:

I've often heard people saying this. And even I say it when someone accuses me of doing something that I didn't do.

I'd say:

Why would I do that?

But what's the grammar behind it?

Is it future from the past tense or something or what?

merged by ColleenV Sep 19 '18 at 18:01

This question was merged with What does why would I do it mean in this context? because it is an exact duplicate of that question.