If your asking a normal question, then it would be -

“Why can’t I sing?”

But that’s not a negative question in the same way as I was talking about.
That would be when you’re asking someone a question.

“Can’t you sing?”

Have fun,
Aidan.

Why I can’t sing?
or
Why can I not sing?

Which is right???

That’s right.

Didn’t you know John wasn’t studying hard?

If I answer “Yes” it means I knew he wasn’t studying hard.
A short answer like this can easily confuse even native speakers though.

Didn’ t you know John wasn’t studying hard ?

Yes.

In this case, generally speaking, the “Yes” means “Yes, I did.”

Am I right ?
Negative questions sometimes confuse me. I guess they might puzzle even native English speakers ???

Sincerely
yuko