な Attributive Copula

In Japanese, the na な particle is actually an attributive copula sometimes.

Grammar

The simplest way to identify the na な copula is with na-adjectives.

The predicative form of a na-adjective ends with the predicative da だ copula, while its attributive form ends with the attributive na な copula. Observe below that you can translate both of these copulas to the English copula "is."

tsuki wa kirei da
月は綺麗だ
The moon is pretty.
- kirei da is the predicate for the subject tsuki.
{kirei na} tsuki
綺麗な月
A moon [that] {is pretty}.
A pretty moon.
- kirei na works like a relative clause qualifying the noun tsuki.

Note that verbs and i-adjectives technically have predicative forms and attributive forms, too. However, their forms are identical:

neko wa kawaii
猫は可愛い
The cat is cute. (predicative kawaii.)
{kawaii} neko
可愛い猫
The cat [that] {is cute}. (attributive kawaii.)
The cute cat.

Only na-adjectives have distinct attributive and predicative forms. Or, more generally, only when you have a phrase that ends in the da だ copula you have to change it to something else (na な) when it comes before a noun.

Although this sounds very easy at first, it's trickier than you may imagine.

Dislocation

In general, you can't use the predicative copula da だ before a noun, only the attributive copula na な.^{(Nightingale, 1996:32)}

*{kirei da} tsuki
綺麗だ月
(this should be wrong, because you should use na な here.)

However, if the sentence is a dislocation, then it's alright:

kirei da, tsuki
綺麗だ、月
(dislocation.)
- tsuki, kirei da
  月、綺麗だ
  (...same as...)
- tsuki wa kirei da
  月は綺麗だ
  The moon is pretty.

This is fine because the matrix clause kirei da is in the predicative, so it can have the predicative copula.

dare da omae?
誰だお前？
(emotive dislocation)
omae wa dare da?
お前は誰だ？
Who are you?

Homonyms

The na な copula can only come before nouns. If you see it coming at the end of the sentence, then it's something else. It's the na な sentence-ending particle, which is different from the copula, and can have multiple functions:

oishii na!
美味しいな！
[It] is tasty, huh! (agreement.)
taberu na!
食べるな！
Don't eat [it]! (negative imperative.)
ayamari na
謝りな
Apologize. (positive imperative.)

Past Form

The past form of the predicative da だ, "is," is datta だった, "was." The past of na な, "is," is datta だった, too. In other words, datta だった can be used both predicatively and attributively, unlike da だ or na な.

tsuki wa kirei datta
月は綺麗だった
The moon was pretty.
{kirei datta} tsuki
綺麗だった月
The moon [that] {was pretty}.
The once pretty moon.

Light Nouns

In Japanese, there are many words that are syntactically nouns, but translate to expressions in English. For example, toki 時, "time," often translates to "when" if it's qualified by something.

So, to say "the time [when] {something is something}," you'll have to change da だ to na な. Observe:

hima da
暇だ
[I] am free, (in the sense of schedule.)
I've got nothing to do.
{hima na} toki dou sureba ii
暇な時どうすればいい
What doing is good the time [when}{[I] am free}.
What should [I] do when {I've got nothing to do}.

Nominalization

The most common instance of this exchange is the compound nano なの. This is the na な copula plus the no の nominalizer, which, too, acts like a noun syntactically.

This no の nominalizer is the no の sentence-ending particle used to ask questions. Observe:

{sore wo taberu} no?
それを食べるの？
Are [you] going to eat that?
{oishii} no?
美味しいの？
Is [it] tasty?

Manga: Erased, Boku dake ga Inai Machi 僕だけがいない街 (Chapter 7, 失われた時間)

baka nano?
バカなの？
Are [you] stupid?
- Original phrase:
- baka da
  馬鹿だ
  [You] are stupid.

The no の particle can combine with the de で particle to form node ので, which sometimes translates to "because."

{kawaii} node suki da
可愛いので好きだ
[It] is liked because {[it] is cute}.
I like it because it's cute.

If the reason before "because" ends in da だ predicatively, you end up with nanode なので.

{suki na} node kawaii
好きなので可愛い
[It] is cute because {[it] is liked}.
It's cute because I like it.

This nanode なので is sometimes contracted to nande なんで.

Note that another word that translates to "because" is kara から. This word is not a noun, but a conjunction, therefore you don't use the attributive copula with it.

kawaii kara suki da
可愛いから好きだ
(same meaning as before.)
suki da kara kawaii
好きだから可愛い
(same meaning as before.)

The no の particle can also combined with the ni に particle, forming noni のに, and consequently nanoni なのに, which can translate to "even though."

konna ni yasui noni?
こんなに安いのに？
Even though [it] is this much cheap?
Even though it's so cheap?
{konna ni kirei na} noni?
こんなに綺麗なのに？
Even though [she] is this much pretty?
Even though she's so pretty?