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In their answer to the question Usage of nan desu?, Earthliŋ♦ described the な in the ~なんだ-construction to be "the inflection of the copula だ, if you like".
I guess this is the same inflection that is used when a な-adjective directly modifies a noun. My question is, why is an inflection used and not just だ by itself? By extension, what is the reason why on the other hand, the そう used to report hearsay requires だ (as opposed to, say, な) when used after nouns and な-adjectives?

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The な after -na adjectives is indeed the same な that appears in the explanatory sentence ending なんだ.

This gets long. I tried creating a "TL;DR" short version, but I couldn't figure out how to really answer your question without including the details. :)

なんだ analysis

Let's break this down.

  • なんだ
  • な の だ
    • な: the "adnominal" form of the copula, as described in modern grammars. This is added after -na adjectives and nouns in this specific kind of construction, as in 本なんだ or 綺麗なんだ. More about な below in the next section.
    • の: the basic possessive particle. Also used to create a kind of clause that nominalizes (makes a noun out of) the preceding phrase, similar to the use of the "-ing" ending in English.
      Examples:
      • XXXするは ("as for doing XX [as an act, as the topic of this sentence]...")
      • XXXを食べるを ("eating XX [as an act, as the object of some other verb]...")
    • だ: the copular "to be" verb. Can be です if the social circumstances of the sentence require a more "polite" or outgroup-facing statement.

The basic meaning of XXなんだ is "it's XX, it's that [whatever we're talking about] is XX". This is used as an explanatory or emphatic statement. This intent is commonly expressed in English more by tone of voice and stress.

な and だ derivation

Now, why would Earthliŋ say the な was "the inflection of the copula だ, if you like"?

な arose as a contraction of にある. にある is particle に + ある, which used to be the copular "to be" verb in Classical Japanese and older. The phonetic progression:

  • にある →
  • なる →

The に particle here is analyzed by some linguists as the gerundive or stem form of an ancient copular "to be" verb, that gave rise to the modern に particle, なる "to become" verb, possibly even the ぬ verb ending in

だ meanwhile arose as a contraction of であり. であり is particle で + あり. The phonetic progression:

  • であり →
  • であ →

This is simplified, and there are various wrinkles. In some parts of Japan, for instance, であ or de a became more like dya which then affricated (added a friction sound) to become じゃ.

More about ある versus あり below.

Grammar

You also ask, "why is an inflection used and not just だ by itself?"

Basic affirmative statements: nouns

The basic affirmative statement formation pattern is essentially just だ as an affirmative statement: "it [whatever we're talking about] is [whatever comes before the だ]".

Words that are nouns can be followed immediately by だ: 本【ほん】だ、車【くるま】だ、茄子【なすび】だ.

Explanatory affirmative statements: verbs

However, you can't use だ immediately after a verb. It would be like saying, in English, "It is [VERB]." Examples: 食べるだ、話すだ → "[TOPIC that we're talking about] is eat, [TOPIC] is talk." That doesn't work in either language.

To make things work in English, we have to change the verb into a noun by using the "-ing" form: "[TOPIC that we're talking about] is [the act of] eating, [TOPIC] is [the act of] talking." The way to change the verb into this kind of noun in Japanese is to add の after it: 食べるのだ、話すのだ.

Say, someone says "what should I be doing now?" A friend might reply "You should be eating." → 食べるのだ.

Explanatory affirmative statements: nouns

Again, to make a basic affirmative statement with a noun ("it's a [NOUN]"), you just say [NOUN]だ.

To make an explanatory statement with a noun ("it's that it is a [NOUN]"), you'd have to use the のだ ending -- but you cannot just say [NOUN]のだ, as that winds up meaning "[whatever we're talking about] is the noun's / belongs to the noun".

So how do we clarify this?

Technically speaking, the phrase before the の here must be in the 連体形【れんたいけい】 or "adnominal" or "noun-modifying" form. (Personally, I suspect this grammatical requirement for the adnominal before the の might be because the original construction included the noun 事 ("fact, act of") between the verb and the particle.)

In modern Japanese, the 連体形【れんたいけい】 and 終止形【しゅうしけい】 or "terminal, sentence-ending, or plain form" are the same thing for almost all verbs. The one clear exception is だ, where the plain form だ is different from the adnominal form な.

This is ultimately because of the way that な and だ evolved. Above, I mentioned that な came from にある, and だ from であり. あり is the terminal or plain form of the Classical Japanese copular "to be" verb, and ある is the adnominal form. So even though most verbs have identical plain and adnominal forms, the modern copular "to be" verb still differentiates, using な for the adnominal and だ for the plain.

So to clarify that we're not talking about something belonging to the noun, but instead talking about the noun, and because the explanatory のだ requires that the preceding phrase end in the adnominal form, we insert this な.

Grammar: examples

Here are some examples of when to, and when not to, insert the な to make an explanatory affirmative statement.

  • Verbs:

    • 話すのだ -- ✔
      The verb is already in the adnominal, so we can just add the explanatory のだ.
    • 話すなのだ -- ✖
      The verb is already in the adnominal, so the な is redundant. It's maybe a bit like saying "it's that it's that [TOPIC that we're talking about] is [the act of] talking."
  • Nouns:

    • 本のだ -- ✖
      Since the phrase before the の is not in the adnominal form, the の acts as a simple possessive. So this winds up meaning "[TOPIC that we're talking about] is the book's / belongs to the book".
    • 本なのだ -- ✔
      Since the phrase before the の is in the adnominal form, the のだ can function as the explanatory affirmative.
  • -i Adjectives:
    Remember that these can function grammatically like verbs. Also, technically speaking, the plain and adnominal forms of -i adjectives have also fused in the modern language, so we can use the same pattern as for verbs.

    • いいのだ -- ✔
      The adjective is already in the adnominal, so we can just add the explanatory のだ.
    • いいなのだ -- ✖
      The adjective is already in the adnominal, so the な is redundant. It's maybe a bit like saying "it's that it's that [TOPIC that we're talking about] is [the fact of being] good."
  • -na Adjectives:
    Remember that these can function grammatically like nouns (and sometimes can also be nouns), so we can use the same pattern as for nouns.

    • 永遠のだ -- ✖
      Since the phrase before the の is not in the adnominal form, the の acts as a simple possessive. So this winds up meaning "[TOPIC that we're talking about] is eternity's / belongs to eternity".
    • 永遠なのだ -- ✔
      Since the phrase before the の is in the adnominal form, the のだ can function as the explanatory affirmative.

There's a lot to get across here to really explain what's going on. If the above is unclear or confusing, please comment and I can rework as needed.

  • Thank you for the passionate response! There is just one thing that I'm still unsure about: Why is the sentence-ending form だ used between nouns / な-adjectives and the そう used to report hearsay? If I may mention my own suggestion: Is it because this そう is more kind of particle than anything? It strongly reminds me of と which also likes to take だ in front of it (きれいだと思います). – Kaskade Apr 11 at 20:42
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    You're mostly right. そう is classed in monolingual JA dictionaries as a 助動詞 or "auxiliary verb", a kind of grab-bag of oddments that grammarians haven't quite known how to classify. In functional terms, そう 1) attaches to the stem or 連用形 form of verbs and adjectives to describe "seems like", and 2) attaches to the terminal form of verbs and adjectives to describe "is apparently like" as reported information (i.e. something heard from someone else, as opposed to directly observed). Some sources say it attaches to the adnominal, but I haven't seen that, and for nouns, the copula must be だ, not な. – Eiríkr Útlendi Apr 11 at 23:06

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