When do you add na to adjectives

So I'm learning Japanese on Duolingo, and there's a unit where they start adding na to the end of adjectives, for example kirei becomes kireina, and they didn't even explain anything. Then, after that unit, they seemingly forgot about it, and stopped doing it. Now I only encounter it in the Practice Hub. I tried reading about it online, and it said that you add na when adjectives modify nouns, but Duolingo doesn't do that (aside from that one unit). I'm really confused, and I feel like I'm missing something. Thank you.