-아/어서 (lý do)
because, so
2-daraja · Bogʻlovchi
Izoh
Gives a reason or cause. ㅏ/ㅗ stems take 아서, others 어서, 하다 → 해서; nouns take (이)라서. No past tense before it (not 갔어서), and the second clause cannot be a command or suggestion (-(으)세요, -(으)ㅂ시다).
Misollar
배가 아파서 병원에 갔어요.
I had a stomachache, so I went to the hospital.
길이 막혀서 늦었어요.
I was late because of the traffic.
휴일이라서 사람이 많아요.
It's a holiday, so it's crowded.
TOPIK izohi
TOPIK I Reading 34–39; TOPIK II Reading 1–2 and 3–4 (paraphrased by -기 때문에, -는 바람에).