also: んです
explanatory/emphatic sentence ending
Attaches to the plain form of verbs, adjectives, and nouns to provide an explanation, reason, or emphasis. Often used when giving or asking for clarification, or when stating something with a sense of background understanding. The ん in んです is a contracted form of の.
Why were you late? (seeking explanation)
I have an exam tomorrow, you see. (providing background)
Is it that he hasn't come yet? (polite, seeking clarification)
から explicitly marks a stated reason or cause ('because'), whereas のです adds an explanatory or emphatic tone without necessarily spelling out the reason.
んです is the contracted explanatory form used after verbs and i-adjectives, while なんです is the parallel contracted form used after nouns and na-adjectives.
Combination of the nominalizer の and the copula です. The contracted form んです derives from the reduction of の to ん in colloquial speech.