-으니까

V + (으)니까 1. This indicates background or situation, especially leading the speaker’s discovery. (I did~ and I realized~)

한국에 도착하니까 8시였어요.

When I arrived in Korea, it was 8 o'clock.

김치를 먹어보니까 많이 매웠어요.

When I tried kimchi, it was very spicy.

TV를 트니까 뉴스가 나왔어요.

When I turned on the TV, the news was on.

• - 는데 vs -으니까

Both are used to suggest the background of following clause.

-는데	-으니까
한국 도착했는데 8시였어요. I arrived Korea, and it was 8.	한국에 도착하니까 8시였어요. I arrived Korea, (and I saw, realized) I arrived at 8.
날씨가 지금은 좋은데 나중에 비가 올 거예요. O The weather is good now, but it will rain later. → Two clauses don’t have to be really related. It’s just a background of why I say.	날씨가 지금은 좋으니까 나중에 비가 올 거예요. X → While 으니까 gives background information, it is used when the speaker realizes or notices the following statement based on the preceding clause. Therefore, it cannot be used in the sentence above.

V/Adj/이다,아니다 + (으)니까 2. This indicates the reason, cause, or basis for judgment. (because~ so I think~)

시간이 없으니까 빨리 가세요.

Since there's no time, please go quickly.

제가 곧 시험이 있으니까 다음에 만나요.

Since I have an exam soon, let's meet next time.

책을 많이 읽으니까 눈이 아파요.

Because I read a lot of books, my eyes hurt.

• -어서 vs -으니까

Both mean ‘because’.

어서	으니까
-어서 can’t be used for imperative and suggesting closing. 시간이 없어서 빨리 가세요 X 시간이 없어서 빨리 갈까요? X	-으니까 can be used for imperative and suggesting closing. 시간이 없으니까 빨리 가세요. O 시간이 없으니까 빨리 갈까요? O
배가 고파서 밥을 먹었어요. I ate becaue I was hungry. 술을 많이 마셔서 건강이 나빠요. He drinks a lot, so he’s unhealthy. → General statement of ‘because’. (In declarative sentences, -어서 is more common.)	배가 고프니까 밥을 먹었어요. (of course) I ate, becaue “I was hungry.” 술을 많이 마시니까 건강이 나빠요. It’s because he drinks a lot, he’s (obviously) unhealthy. → -으니까 is used when you want to highlight the preceding clause as the reason, or when you want to show your own subjective judgment.

• Tense

meaning 1: always base form

TV를 틀었으니까 뉴스가 나왔어요 X → 트니까 O

meaning 2: no strict tense restrictions