있었던 건

what was there

→ 있다

phrase Rare #27,157

what was there 있었던 means “was” or “there was”; 건 is a contraction of 것은, marking the phrase as the topic. Depending on context, it can refer to a person or thing that was somewhere, or to the fact that something existed.

KO: 그때 거기 있었던 건 나뿐이야.

EN: I was the only one there then.

하지만 거기 있었던 건 안톤이 아니잖아요

But Anton wasn’t the one who was there.

— 데이 워치 (2006)

누구 엉덩이에 문신이 있었던 건 분명 기억나지 않았다.

I definitely didn't remember whose butt had a tattoo.