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if I'm not mistaken, そう is 然う (an adverb) but I don't see any rules about adverbs when I look up the rules for the grammar pattern のだ. they only say that な should be added to na-adjectives/nouns, nothing is said about adverbs at all.

Am I wrong about the noun/adjective distinction? About the grammar of そうなんだ? A possible omission of a noun?

Sante Kyaku
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  • Any time you have the んだ・のだ explanatory ending, the piece beforehand must be in the attributive form or 連体形【れんたいけい】. Have a look at [this answer](https://japanese.stackexchange.com/questions/90445/%e5%b9%b4%e3%81%aa%e3%82%93%e3%81%a0%e3%81%8b%e3%82%89%e7%84%a1%e7%90%86-how-does-one-parse-this-colloquial-sentence/90455#90455) and the others linked from there. – Eiríkr Útlendi Aug 18 '22 at 21:25

1 Answers1

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That is そうだ + んだ. The copula, だ, always becomes な when the explanatory の/ん follows.

user3856370
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