I had a review today with the sentence: That is awful.
それは……です。[たいへん]
Since it was about な-adjectives, I filled in:
たいへんな
But that seems to be wrong, it just has to be たいへん.
Where did the な go?
Can someone explain? I seem to be missing something here. Thanks!
I’ve been trying to think of an easy way to put this into words but man, teaching is super tough haha. The adjective here isn’t directly modifying a noun, if that makes any sense. In the first example sentence on Bunpro, you can see that 綺麗 is placed before 絵, saying that is a pretty painting.
I believe some of the initial confusion comes from how the last example sentence is created, because some people say “well, isn’t 彼 a noun and getting talked about too?” Someone better with words can explain this in more detail if needed, but I always thought about the Tae Kim resource where they say that an adjective can directly modify a noun if it comes right before it in the sentence. I’ll link the resource below, but I’d also recommend any of the other ones in the resource tab if Tae Kim is too dry for you (a common complaint!) That said, there’s lots of example sentences in there that showcase the difference of something being modified vs. a general description, hope that helps! If not lemme know!
Adjectives – Learn Japanese (the video here, especially everything from the 1:40 mark onwards, does about a 400000x better job than I ever could.)
I am a beginner too, so take my answer with a grain of salt.
The な particle is used to join a な adjective to a noun, e.g. to say “terrible/serious news” would be 大変なニュース. This is different from saying “the news is terrible,” in which case the adjective isn’t attached to a noun, so no な. Instead, the fact that it’s a な adjective means you need だ/です to mean “is”, whereas an い adjective already acts like a verb meaning “it is [adjective]”.
Apart from what others have said, I’ll add another example as a way to think about it: の, the possessive particle.
Similar to possession, “my book” for example, 私の本（わたしのほん）, I am describing the book. Whose book is it? It’s my book. What book is it? It’s my book. The の informs you of the description.
な works in a similar way, and that’s often why when you come across grammar rules, な adjectives and nouns almost always work similarly. (You’re not far enough to have seen that yet, but the pattern will gradually appear!)
It is a book. 本です。ほんです。
It’s my book. 私の本です。わたしのほんです。
It’s an important book. (な adjective) 大切な本です。たいせつなほんです。
The book is important. 本は大切です。ほんはたいせつです。
For the latter sentence, たいせつ is not describing anything specific. As for the book, it is important. Unlike the first sentence, where たいせつ is directly describing the book.
As Kfrl said, the い is the bit of the odd one out, as it doesn’t have any additional attachment to function in either situation. But you’ll see later that it has other forms it takes depending on the situation.
It’s an interesting book. (い adjective) 面白い本です。おもしろいほんです。
The book is interesting. 本は面白いです。ほんはおもしろいです。
Because of this though, い adjectives get to be special, and don’t need a final です・だ to complete the sentence, whereas な adjectives do. (I’m not sure if you’re this far yet, but I wanted to mention it just in case, but no stress if you don’t understand!)
本は大切。ほんはたいせつ。
Wrong
本は面白い。ほんはおもしろい。
Okay
Don’t stress if it takes a while to click. Japanese grammar is the most far removed from English, and a lot of languages really, there is, so it can take some getting used to. Hopefully these posts helped you a bit though. Keep at it! 
In one way な is just an adjectival version of だ, and です is the polite form of だ. That is where it went.
Thanks all! Makes more sense now.