のです

expression

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.

• どうして遅おくれたんですか。

  Why were you late? (seeking explanation)

• 明日あしたは試し験けんがあるんです。

  I have an exam tomorrow, you see. (providing background)

• 彼かれはまだ来こないのですか。

  Is it that he hasn't come yet? (polite, seeking clarification)