When we talk, we sometimes tell others additional information about the situation or reason. In English, we usually use separate sentences. However, in Korean, you can use -는데/-(으)ㄴ데, or -(으)니.
어제 집에 가는데 친구를 만났어.
I met a friend when I was coming back home yesterday.
시간도 많은데 느긋하게 해.
We have a lot of time. Relax and do it slowly.
학교에 와 보니 아무도 없었어.
When I arrived at school, no one was in it.
먹고 보니 내 동생 거더라고.
I realized when I finished eating it. It was my sister’s.
Let’s check grammar rules.
Past tense is OK for -는데/-(으)ㄴ데. But don’t use others for -는데/-(으)ㄴ데.
First, you can use past tense endings with -는데/-(으)ㄴ데. But it is usually awkward to use future tense endings.
지난 주에 학교 축제가 있는데 사람은 별로 없었다. (x)
지난 주에 학교 축제가 있었는데 사람은 별로 없었다. (o)
Last week, there was school festival. But there wasn’t many people.
다음 주에 학교 축제가 있겠는데 사람이 별로 안 올 것 같아. (x)
다음 주에 학교 축제가 있는데 사람이 별로 안 올 것 같아. (o)
There is school festival next week. But it seems that many people won’t come.
이 문제는 모르겠는데 누구한테 물어 보지?
I cannot solve this problem. Who should I ask about it?
As for the last example, -는데 here doesn’t mean background information. It’s a reason. So, you can use “겠”.
You cannot use -(으)니 with tense endings.
일을 마치고 시계를 봤으니 벌써 12시가 넘었더라. (x)
일을 마치고 시계를 보겠으니 12시가 넘어 있을 거야. (x)
일을 마치고 시계를 보니 12시가 넘었더라. (o)
I saw the clock after finishing my job. It was after 12 o’clock.
No restriction in subjects
Second, there’s no restriction in subjects.
길가에 큰 벚나무가 하나 있는데, 예뻐서 사람들이 사진을 많이 찍고 간다.
There’s a big cherry tree beside the street. As it’s pretty, many people take a photo of it.
자고 일어나 보니 내 친구가 유명해져 있었다.
One morning I woke up and found my friend famous.
You can use descriptive verbs and 이다 verb. But…
Third, you can use -(으)니 with descriptive verbs and 이다 verb. But in this case, they usually mean “reason” rather than “additional information”.
이제 고등학생이니 진로에 대해 생각해 봐야겠다.
As I became a high school student, I should think about my career.
이게 짧으니 이걸로 하자.
Let’s buy this because it’s short.
It’s OK to use requesting/imperative sentences with -는데/-(으)ㄴ데.
Fourth, you can use -는데/-(으)ㄴ데 with requesting/imperative sentences. But as for -(으)니, you cannot.
백화점에 가니, 같이 가자. (x)
백화점에 갈 건데, 같이 가자. (o)
I’m going to department store. Let’s go together.
백화점에 가니, 빨리 준비해. (x)
백화점에 갈 건데, 빨리 준비해. (o)
Get ready. Quick. We’re goint to department store.
Don’t use -(으)니 with negative sentences.
Fifth, you can use negative sentences with -는데/-(으)ㄴ데. But you cannot with -(으)니.
오늘 안 가니 괜찮아? (x)
오늘 안 가는데, 괜찮아? (o)
I’m not going today. Is it all right?
내일은 안 오니, 준비 다 됐지? (x)
내일은 안 오는데, 준비 다 됐지? (o)
They won’t come tomorrow. But you’re completely ready. Right?