I am not able to frame a question, but here is the situation: in your family your father is doubting your knowledge and he is asking you very simple questions.

I want to reply him by saying why would I not know such a simple thing. It is very easy. So is this sentence grammatically correct?

"Why would I not know?"

I want to understand the grammar behind this. This is the only question I want to answer. I have already tried but not been able to portray it.