What does the ending “-거든” mean in Korean?

The ending -거든 (polite -거든요) states a fact the other person seems to have forgotten, so it works as a gentle correction or an explanation. 나 한국인이거든? is “I’m Korean, you know,” and the rising tone gives it an edge. Said flat, -거든 just gives a reason: 배고프거든요, “it’s just that I’m hungry.”

In the clip, Jisu is dumped by Jinho, who says he wants to meet 한국 여자. She answers 야, 나 한국인이거든? (“Hey, I’m Korean!”), and the joke is that she is exactly what he says he wants. The breakup line before it, 우리는 헤어지는 게 좋을 거 같애 (“I think we should break up”), uses the soft -는 게 좋을 것 같다 to say something harsh.