きれいな and おそい are adjectives, but きれいに and おそく are adverbs. To turn a な-type adjective into an adverb, change the な to に. To turn a い-type adjective into an adverb, change the い to く. ex. このへやはきれいです → This room is clean. このへやはきれいにしてください → Please make this room clean (Please clean this room) わたしはおそいです → I am late わたしはおそくきました → I came late.

おそい = adjective - modifies a noun (or noun equivalent) おそく = adverb - modifies anything else Keep on studying. You'll figure it out.

good question