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?
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2022-08-18T17:39:22.823
95876
2022-08-18T19:34:20.143
32056
question
particle-の explanatory-の
Why does そうなんだ have a な?
That is そうだ + んだ. The copula, だ, always becomes な when the explanatory の/ん
follows.
CC BY-SA 4.0
2022-08-18T19:34:20.143
95881
2022-08-18T19:34:20.143
7944
95876
answer