んです Conjugation


We recently learned んです form in class and I think I understand when to use it.
But I'm a little unclear about how conjugation of it works.

んです itself doesn't get conjugated, right? You conjugate the
verb/noun/adjective that comes before it.

  * 頭がいたいんです。
  * 風邪をひいたんです。
  * 本が好きなんです。

And, as with な adjectives, な also gets added after nouns. But how would this
work in the past tense?




> んです itself doesn't get conjugated, right?

んです（≂ のです; particle の + copula です） conjugates to:

んです、んでした、んでしょう、(んでして、) -- polite  
んだ、んだった、んだろう、んで、んなら -- plain

> And, as with な adjectives, な also gets added after nouns. But how would this
> work in the past tense?

You replace な with だった, as in:

花 **な** んです。→ 花 **だった** んです。

