-아서/어서
-aseo/eoseo
The upper basin spills into the lower one: -아서/어서
The picture Water running down a slope from one basin into the next. You cannot pull the two apart. -아서 lets a reason flow down on its own, while -(으)니까 sets the reason down in front of the listener — which is why orders and suggestions take -(으)니까: 비 오니까 우산 챙기세요, not 비 와서 우산 챙기세요.
Where the picture shows up in -아서/어서
-
the reason that flowed in
늦어서 죄송합니다.
neu-jeo-seo joe-song-ham-ni-da
Sorry I'm late.
Holding up a watch long past the meeting time and bowing your head — the lateness runs straight on into the apology.
-
what the jam caused
길이 막혀서 못 갔어요.
gi-ri ma-kyeo-seo mot ga-sseo-yo
The traffic was bad, so I couldn't make it.
Sitting in a car stopped dead in packed traffic, while the meeting spot stays empty.
Never put a past-tense marker before -아서/어서. 막혔어서 is wrong.
-
go there, then do it there
은행에 가서 돈을 찾았어요.
eun-haeng-e ga-seo do-neul cha-ja-sseo-yo
I went to the bank and took out some cash.
Pushing through the bank door and taking the cash at the counter inside — the place you went to carries on.
-
staying in that position
앉아서 기다리세요.
an-ja-seo gi-da-ri-se-yo
Please have a seat and wait.
Lowering yourself into the chair and waiting from exactly that position.
-
the feeling it produced
만나서 반갑습니다.
man-na-seo ban-gap-seum-ni-da
Nice to meet you.
Two hands clasped, and a smile spreading out of the handshake.
A command or suggestion cannot follow -아서/어서. Say 늦었으니까 빨리 오세요, not 늦어서 빨리 오세요.
All of it in one picture
-아서/어서 is two basins on a slope. Let lateness overflow into an apology and you have a reason; let a jammed road run down into not making it and you have a result; let feet that reached the bank draw cash right there and you have one place carrying into the next. Let a seated posture flow on into waiting and it is holding a position; let meeting someone well up into gladness and it is a feeling pouring out of that moment. So the two basins -아서 joins cannot be pulled apart — remove the first and the water stops, leaving the second clause nothing to stand on.