んです Conjugation

Question

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?

Answer 1

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。