Are な and だ both copula verbs?


What is the grammatical relationship between these two words?

Are they just different words expressing the same concept, is one the
derivation of the other or is a third option true?




**だ is a conclusive copula** , etymologically a contraction of で＋ある. It is
used sentence-finally (hence the name "conclusive"). The uncontracted form is
still available in Modern Japanese, but it's somewhat different in
distribution and more formal. だ isn't actually a verb―it cannot stand alone
and doesn't inflect like a verb. で＋ある isn't a verb either, but it _is_ a
construction containing the verb ある. The copular で is a contraction of
literary にて.

**な is an adnominal copula** , etymologically a contraction of なる from に＋ある.
It is used before nouns (hence the name "adnominal"). The uncontracted form なる
is now very limited and―outside of the literary register―usually only appears
with specific words such as 聖なる that can now be viewed as lexical exceptions.

The two can be now viewed as forms of the same thing. Etymologically, the
difference is that だ contains て, but な does not:

[code]

    　（に＋て）＋あり　→　にて＋あり　→　で＋ある　→　であ　→　だ　（conclusive copula)
    　　に　　　＋あり　→　に　＋あり　→　なり　　　→　なる　→　な　（adnominal copula） 　
    
[/code]

Etymologically speaking, both だ and な contain the Old Japanese **copula
infinitive に** `n-i`, which is related to **the adnominal copula の** `n-o`,
which was present in Old Japanese and is still around today:

[code]

    　（n-i＋て）＋あり　→　にて＋あり　→　で＋ある　→　であ　→　だ　（conclusive copula)
    　　n-i　　　＋あり　→　に　＋あり　→　なり　　　→　なる　→　な　（adnominal copula） 　
    　　n-o　　　　　　　　　　　　　　　　　　　　　　　　　　　→　の　（adnominal copula)
    
[/code]

In other words, all three forms descend from the same Old Japanese defective
copula `n-`. Note that in traditional grammar, the adnominal copula の is not
analyzed as a copula and is instead treated as a use of the genitive case
particle の. You can find it in monolingual Japanese dictionaries listed as
expressing 同格の関係 ("an appositive relationship").

* * *

In modern Japanese, we find all three forms in alternation, and they can all
be considered forms of the same thing. Generally speaking, **だ** is used
clause-finally ("conclusively"), while **な** and **の** are used before nouns
("adnominally"). Specifically:

  1. **な** is used following な-adjectives (called 形容動詞 in traditional grammar)
  2. **の** is used following nouns under most circumstances (including の-adjectives)
  3. **な** is used following nouns before certain grammatical words such as の

So we find:

[code]

    　きれい **だ**　〜　きれい **な** 花　　　　（きれい is a な-adjective）
    　　病気 **だ**　〜　　病気 **の** 人　　　　（病気 is a の-adjective, a special type of noun）
    　　　弟 **だ**　〜　　　弟 **の** 太郎　　　（弟 is a regular noun）
    　　病気 **だ**　〜　　病気 **な** のだ　　　（な is used before certain grammatical words such as の）
    
[/code]

In traditional grammar, の-adjectives are simply considered nouns, and for our
purposes here we can follow the same line of thinking. The boundary between
the noun and な-adjective categories is rather fluid in any case, and many
words belong to both categories.

Note that in traditional grammar, だ is analyzed as a separate word when it
follows a noun, but an inflectional ending when it follows a な adjective
(形容動詞). Most modern grammars depart from traditional grammar on this point and
present だ as a separate word in all cases, as I've done here.

* * *

References, by request:

  * For more historical information about だ, な, and の, see Frellesvig's 2010 _A History of the Japanese Language_ or Vovin's 2005, 2009 _A Descriptive and Comparative Grammar of Western Old Japanese_. 
  * For a brief introduction to だ, な, and の in Modern Japanese, see Iwasaki's 2013 _Japanese: Revised edition_. 
  * For a detailed modern analysis, see Martin's 1975 _A Reference Grammar of Japanese_ , though he uses rather different terminology―he calls の-adjectives "precopular nouns" and な-adjectives "adjectival nouns".
  * For a quick online reference, see [大辞林's entry for だ](http://www.excite.co.jp/dictionary/japanese/?search=%E3%81%A0&match=beginswith&itemid=DJR_da_-070), which covers its etymology and the use of adnominal な before words like の.

