intermediate
Gives a reason based on the speaker's own judgement or discovery.
Usage
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Stem ends in a vowel or ㄹ → 니까 (가다 → 가니까, 살다 → 사니까).
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Stem ends in a consonant → 으니까 (먹다 → 먹으니까).
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Unlike 아/어서, it takes tense: 갔으니까, 먹었으니까.
Example:
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위험하니까 조심하세요. (It's dangerous, so be careful).
Nuance
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This is the reason form you must use before a command, suggestion, or invitation: 시간 없으니까 빨리 가자!
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It frames the reason as the speaker's own conclusion, which is why it can sound blunt or self-justifying in an apology. For 죄송합니다, always pair with 아/어서.
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It also marks discovery: 집에 가니까 아무도 없었어요. (When I got home, [I found that] nobody was there).
In Context
배고프니까 밥 먹으러 가요.
I'm hungry, so let's go eat.
비가 오니까 우산을 가져가세요.
It's raining, so take an umbrella.
제가 했으니까 걱정하지 마세요.
I did it, so don't worry.