んです details

write + it's that…

kaku n desu

it's that…; you see — marks the utterance as an explanation

遅おくれてすみません。電車でんしゃが止とまっていたんです。

okurete sumimasen. densha ga tomatte ita n desu.

Sorry I'm late — the trains had stopped, you see. neutral

その荷物にもつ、どうしたんですか。

sono nimotsu, dou shita n desu ka.

What's the story with all that luggage? neutral

formal
colloquial
question

Examples

gives the explanation the context calls for — answering a real or implied "why"

遅おくれてすみません。電車でんしゃが止とまっていたんです。

okurete sumimasen. densha ga tomatte ita n desu.

Sorry I'm late — the trains had stopped, you see. neutral

ごめん、今日きょうは行いけない。熱ねつがあるんだ。

gomen, kyou wa ikenai. netsu ga aru n da.

Sorry, I can't make it today. I've got a fever. casual

asks for an explanation — んですか probes the story behind what the speaker has noticed

その荷物にもつ、どうしたんですか。

sono nimotsu, dou shita n desu ka.

What's the story with all that luggage? neutral

あれ、まだ帰かえらないの？

are, mada kaeranai no?

Huh, you're not going home yet? casual

Formation

{V}んですattaches to dictionary form
plain clause + んです（N・na-adj はなんです）attaches to plain clause

Variant notes

のです — the uncontracted form — written language and careful formal speech
んだ — plain-style counterpart; question-final 〜の？ is gender-neutral, assertive 〜の。 skews soft/feminine

Notes

Nouns and na-adjectives insert な: 学生[がくせい]なんです, 好[す]きなんです. Tense stays inside the clause — 行[い]ったんです. (行くんだった／んでした exists but is a different pattern: regret, "should have gone.")

Related patterns

Similar

ので
Both ride the explanatory の — ので hands the reason to the NEXT clause (止まっていたので遅れた), のです/んです offers the whole sentence AS the owed explanation (止まっていたんです); one connects, the other concludes.