also: なんだ
auxiliary
it is that ...; the thing is ...; I assure you that ...
Explanatory or assertive sentence-ending auxiliary. Used after nouns and na-adjectives to give a reason, explanation, or to add emphasis. Equivalent to のだ after verbs and i-adjectives.
See also: のだ
He is a student (you see).
auxiliary
did not ...
Only with reading なんだ
Kansai dialect negative past form, attaching to the -nai stem of a verb. Equivalent to standard ～なかった.
I didn't go yesterday. (Kansai dialect)
That's the problem (I'm telling you).
なのです is the polite/formal version of なのだ, used in the same explanatory role but in polite speech.
Combination of the copula だ (or its attributive form な) and the explanatory particle の, with だ added for assertion. The Kansai dialect negative sense is a contraction of なかった.