そうなんです

interjection

yes; that's right; that is so

Colloquial confirmation or agreement, often used when the speaker realizes or acknowledges something. The なんです adds explanatory or emphatic nuance.

• あ、そうなんです。私わたしもそう思おもいます。

  Oh, yes, that's right. I think so too.

• そうなんです。実じつは昨日