Why is だ used for な adjectives?


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?




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](http://www.ic.nanzan-u.ac.jp/LINGUISTICS/staff/munakata_takashi/pdf/munakata_fajl4_complementizer.pdf)
sums up multiple positions and seems to give a pretty reasonable analysis, but
I'm still learning, so I'm not exactly sure :-)




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](http://www.ic.nanzan-u.ac.jp/LINGUISTICS/staff/munakata_takashi/pdf/munakata_fajl4_complementizer.pdf)
sums up multiple positions and seems to give a pretty reasonable analysis, but
I'm still learning, so I'm not exactly sure :-)

