〜んだから

子供(こども)なんだから、仕方(しかた)ないよ。

Kodomo na n da kara, shikata nai yo.

They are a child — nothing to be done.

んだから insists the reason is obvious — it should not need saying

もう大人(おとな)なんだから、自分(じぶん)で決(き)めなさい。

Mō otona na n da kara, jibun de kimenasai.

You are an adult now — decide for yourself.

A command or a piece of advice usually follows, the premise being treated as settled

違(ちが)いは、理由(りゆう)を当然(とうぜん)と見(み)るかどうかです。

Chigai wa, riyū o tōzen to miru ka dō ka desu.

The difference is whether the reason is treated as self-evident.

から merely gives a reason; んだから adds that you ought to have known it