When should I use な in "na-adjectives"?

Question

I'm trying to understand something here, I have the following question I need to answer:

あなたの部屋はきれいですか？

to which I should answer something like

ええ、きれいなです。

Why is the question does not contain the な?

Answer 1

Actually, your question is right and your answer is wrong.

You will only use な after a "na-adjective" if you're modifying a noun/adjective with it, for instance:

あなたの部屋はきれいですか (anata no heya wa kirei desu ka) Is your room beautiful?

はい、きれいですよ (hai, kirei desu yo) Yep, it is beautiful.

きれい is not modifying any noun, therefore there's no need for using な in this.

But if you wanted, you could word your answer like this:

はい、きれいな部屋ですよ！ (hai, kirei na heya desu yo!) Yep, it's a beautiful room!

here we wanna state clearly that it is the room that has the quality of being beautiful, in order to do that we, then, need to attach な after the adjective, therefore:

きれいな部屋 (na-adjective + na + noun)