Why does そうなんだ have a な?


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?




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

