I've always compared な adjectives to some abstract harder type of adjective, and I think I learned formally what the difference is, kind of, but I've forgotten. In any case, I know there are nouns that can act as adjectives, but what is the reason for why な adjectives are used in forms like 大切だと思う, similar to nouns? With other adjectives だ is omitted, so what exactly does the だ mean in this case? What part of speech is it?

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    な is a form of だ. So-called "な-adjectives" behave syntactically like nouns. – Zhen Lin Sep 18 '13 at 23:33
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    How is it a form? In what way? – user3457 Sep 19 '13 at 0:14
  • Also note that not all adjectives are so plainly な or い, there is some overlap (大きい vs. 大きな, etc.). See this thread – jmac Sep 19 '13 at 1:48
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    「な」=「にある」, 「だ」=「である」. – Ignacio Vazquez-Abrams Sep 19 '13 at 2:05
  • @IgnacioVazquez-Abrams This is an excellent little rule. Is it hard and fast, or soft and wobbly? – Jamie Taylor Sep 19 '13 at 14:13

In traditional grammar, words that inflect, called 用言{ようげん}, are given six inflected forms, called 活用形{かつようけい}:

  • 未然形(みぜんけい)   irrealis form
  • 連用形(れんようけい)  continuative form
  • 終止形(しゅうしけい)  terminal form
  • 連体形(れんたいけい)  adnominal form
  • 仮定形(かていけい)   hypothetical form (see note 1)
  • 命令形(めいれいけい)  imperative form

And in traditional grammar, だ is considered a type of 助動詞{じょどうし}. That's a separate part of speech which I'll translate inflectable suffix here. And since that's a type of inflectable word, it too has forms that fit into the six categories above. The two forms you've asked about are the 終止形 and the 連体形, and I'll being focusing on those two only in this answer:

  • The 終止形 is the form that can come at the end of a sentence.
  • The 連体形 is the form that can come before a noun as a modifier.

Consider the following:

Example 1: 海{うみ}が綺麗{きれい}だ

Here, だ is in its sentence final form (終止形), ending a sentence (see note 2). Here, 綺麗だ is a predicate. But what happens if we instead use 綺麗だ as a modifier, putting the noun 海 after it?

Example 2: 綺麗な海だ

It has to take the adnominal form (連体形) instead, because that's the form that can come before a noun. And the 連体形 of だ is な, so that's what you have to use here.

Now I want to address your question about 大切だと思う. Here's what you asked:

With other adjectives だ is omitted, so what exactly does the だ mean in this case?

Well, i-adjectives, called 形容詞(けいようし)in traditional grammar, are also a type of inflecting word. They too have a 終止形 and can end a sentence:

Example 3: 海が美しい

Example 4: 美しい海だ

Again, the first example shows the 終止形, which can end a sentence. The second example shows the 連体形, modifying a noun. The weird thing here is that both forms look the same! That's because modern Japanese no longer distinguishes the two forms for this kind of adjective. Still, we can say that 美しい is a 終止形 in example 3.

So what we can see here is that 美しい forms a complete predicate and can end a sentence. And so can 綺麗だ, as we saw in example 1.

And what about と? It can come after a 終止形, turning a clause into the complement of another verb, such as 思う. In modern analysis, some linguists call this a complementizer (see note 3). Take a look at the following examples:

Example 5a: 綺麗だ
Example 5b: 綺麗だと思う
Example 6a: 美しい
Example 6b: 美しいと思う

As you can see, you need だ in example 5 because without it, you don't have a complete sentence for と to act on. You don't need it in example 6 because you already have a complete sentence with just 美しい. (Of course, *美しいだ is ungrammatical.) In other words, だ isn't omitted with other adjectives. It wasn't present in the first place.

Finally, I'll point out that だ and な are etymologically close to being the same thing, so there's some historical justification for calling them forms of the same thing. The former comes ultimately from にてあり, while the latter comes from にあり. These are almost the same thing; the only difference is that な is missing the て.

Note 1: The 仮定形{かていけい} was called the 已然形{いぜんけい} (realis form) in classical Japanese grammar. It's usually called 仮定形 instead when analyzing modern Japanese because it has fewer uses, being essentially limited to forming hypotheticals.

Note 2: Actually, in traditional grammar, you wouldn't say that this is 綺麗 followed by だ. You'd say it's a single word, 綺麗だ, and you'd say that 綺麗な is a form of 綺麗だ. That's a little strange, since etymologically speaking, it's the same だ or な you use in any other situation. In any case, my answer departs from traditional grammar on this point.

Note 3: There's some debate over whether what exactly the function of と is. This paper sums up multiple positions and seems to give a pretty reasonable analysis, but I'm still learning, so I'm not exactly sure :-)

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    Nice writeup! One thing that might be confusing is that the だ in 終止形 is sometimes dropped (これって大切!) and even before と (lots of Google hits for "大切と思う", even though I personally consider this slightly marked). – dainichi Sep 19 '13 at 2:35

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