-(으)니까
-(eu)nikka
Putting your reason on the table: -(으)니까
The picture Laying a card face up in front of the other person: here is my reason, so let's do this. -(으)니까 sets the grounds down so an order or invitation can follow, while -아서/어서 only flows into a result — an apology goes 늦어서 죄송합니다, never 늦으니까 죄송합니다.
Where the picture shows up in -(으)니까
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the reason you hand over
비 오니까 우산 가져가세요.
bi o-ni-kka u-san ga-jyeo-ga-se-yo
It's raining, so take an umbrella.
Pointing out the window at the falling rain and pressing an umbrella into the other person's hand.
A command cannot follow -어서, but it can follow -(으)니까.
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grounds for doing it together
시간 없으니까 택시 탑시다.
si-gan eop-seu-ni-kka taek-si tap-si-da
We're out of time, so let's take a taxi.
Holding your watch up for them to see, then raising an arm to flag down a taxi.
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what you found on arriving
집에 가 보니까 아무도 없었어요.
ji-be ga bo-ni-kka a-mu-do eop-seo-sseo-yo
I went home and found nobody there.
Opening the front door onto a dark, empty living room — a scene that appears only once you are inside.
It also marks something you discovered only after doing the first action.
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the card under your claim
제가 먼저 왔으니까 제가 낼게요.
je-ga meon-jeo wa-sseu-ni-kka je-ga nael-ge-yo
I got here first, so I'll pay.
Reaching for your wallet first, from the seat you had already been sitting in.
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the reason to put it off
바쁘니까 나중에 얘기해요.
ba-ppeu-ni-kka na-jung-e yae-gi-hae-yo
I'm busy, so let's talk later.
Waving a hand without breaking stride, an armful of papers held against your chest.
Don't use it for apologies or greetings. 늦어서 죄송합니다 is right; 늦으니까 죄송합니다 sounds broken.
All of it in one picture
-(으)니까 is laying one card face up in front of someone. Point at the falling rain and it is grounds for taking an umbrella; hold up your watch and it is grounds for the taxi; put down the fact that you arrived first and it is grounds for paying. Walk in and find the living room dark, and the card is what the going revealed; wave with an armful of papers and the card says let's talk later. That is why an order or an invitation can follow -(으)니까 — you have already put the reason in the other person's hand.