adjective + な or noun + は

Question

I am trying to understand what is the difference between the following:

• adjective + な + noun + verb => きれい な 先生 だ
• noun + は + adjective + verb => 先生 は きれい だ

and when to use it as well, I can't seem to see when to use one or the other

Answer 1

Both are grammatically correct, and share little difference in how to use them as far as I am aware. Breaking it down into specifics:

きれいな先生だ

This is, as you said, [adjective] + な + [noun]. A literal translation would read something like:

"Beautiful teacher."

While your other sentence:

先生はきれいだ

Would read literally as:

"As for the teacher, [he/she/they are] beautiful."

Like Aguijonazo pointed out, this difference is very subtle. With the example words you've provided, the following would read like (in a more natural way):

[adjective] + な + [noun] = A beautiful teacher.

[noun] + は + adjective = The teacher is beautiful.

So, is there a difference? Should one be used over the other? The answer is: most likely not. It probably doesn't matter. In most situations it most likely won't. But there is a slight emphasis on the words depending which route you choose.

Recall, は is a topic marking particle. By saying 先生はきれいだ you are marking 先生 - the teacher - as the topic. To me, in this vain, using は (in the context of speaking to someone else) sounds like trying to start a conversation, while the former is more of a passing comment. Of course this isn't always true, but it's more of the vibe I get.

Context is also a factor, of course. If you and a friend are chatting about some other topic - say, your classes (好きな授業は何ですか？) - and the teacher suddenly walks in, I think it'd be more correct to say 先生はきれいだ, as you are switching the topic to something different. (Although this is admittedly abrupt sounding).

If you're unsure, I'd probably just use 先生(は/が)きれいだ.

_{I'm still learning myself so feel free to correct me if I made a mistake.}