~ㄴ/는/다고 하던데 (~eun/neun/da-go ha-deon-de) – I heard about this (from someone) but/so…
This phrase is basically a combination of ~는대/ㄴ대/대/이래, ~냬, ~으래/래, ~쟤 (reported/quoted speech) and 던데.
You can look at it this way:
~ㄴ/는/다고 하다 + 던데
I heard someone saying/talking/asking about this (before/in the past/some time back), but/so/therefore/thus…
Examples: 한국사람들은 매운 음식을 잘 먹는다고 하던데 왜 현진 씨까 잘 못 먹어요?
I heard (from people) that Koreans eat spicy food well but why does Hyunjin not eat spicy food well?
엄마가 오늘밤은 우리 위해 부대찌개를 준비하는다고 하던데 일찍 집으로 돌아가야지.
I heard (from someone) that Mom will be preparing army stew for us tonight so I should go back home early, right.
언니가 우리 화장품을 사자고 하던데 왜 너무 늦었도 안 보여?
I heard that Unnie suggested that we go buy cosmetics together but why don’t I see her even when it’s so late?
선생님은 숙제를 제출하라고 하던데 지금도 아직 하지 않아서 걱정돼.
I heard that Teacher asked us to hand in our homework but I’m haven’t even done it now so I’m worried.