そうなんだ。

Sōnanda.

Oh, I see. / Is that so?

Word for word

“Oh, so that’s how it is.”

Politeness: Casual

Plain form. Fine with family, close friends and people younger than you. Rude to a stranger or a boss.

When you would say it

Responding to new information from a friend.

Worth knowing

Polite: そうなんですね。