So and because: -아/어서 and 그래서

Reason first, then the result

아파요 — I’m sick. 아파서 병원에 가요 — I’m sick, so I’m going to the hospital. 배고파요 — I’m hungry. 배고파서 밥을 먹어요 — I’m hungry, so I’m eating.

The first half is the 해요 form with 서 where 요 was. It gives the reason, and it always comes before the result. English can say it either way round, “because I’m sick, I’m going” or “I’m going because I’m sick”. Korean has one order.

해요 form, minus 요, plus 서

Korean says rain and snow “come”: 비가 와요 — it’s raining. 눈이 와요 — it’s snowing. As reasons: 비가 와서 집에 있어요 — it’s raining, so I’m at home. 눈이 와서 학교에 못 가요.

Textbooks write this ending -아/어서, because it follows the same vowel choice as the 해요 form. In practice you never need to choose again: take the 해요 form, remove 요, add 서. 와요 → 와서, 먹어요 → 먹어서, 피곤해요 → 피곤해서.

눈 “snow” is spelled exactly like 눈 “eye”.

The reason carries no tense

어제 아파서 못 갔어요 — I couldn’t go yesterday because I was sick. In English both halves are past: “was sick”, “couldn’t go”. In Korean only the last verb says so. The reason stays 아파서, never a past form with 서.

The same again with a new word, 약, medicine: 아파서 약을 먹었어요 — I was sick, so I took some medicine. Korean “eats” medicine: 약을 먹어요.

Late and early

늦어요 means “is late”, dictionary form 늦다. Two adverbs say when you do something: 늦게 late, 일찍 early.

늦게 일어나서 회사에 늦었어요 — I got up late, so I was late for work. 피곤해서 일찍 잤어요 — I was tired, so I went to bed early. Notice the pair in the first sentence: 늦게 goes with another verb, while 늦었어요 is itself the verb.

늦게 is pronounced [늗께], 늦어요 [느저요].

Two sentences: 그래서

비가 와요. 그래서 집에 있어요. — it’s raining. So I’m staying at home. When the reason is a finished sentence, the next one opens with the separate word 그래서, “so”.

Because these are two complete sentences, each has its own tense: 어제 아팠어요. 그래서 학교에 못 갔어요. Compare the one-sentence version, where the past appears once: 어제 아파서 학교에 못 갔어요.

왜요? — just the reason, plus 요

오늘 파티에 못 가요 — I can’t go to the party today. 왜요? — why? 바빠서요 — because I’m busy.

왜요? is 왜 with the polite 요: a complete question in one word. The answer can stop at the reason. But a bare 바빠서 would sound unfinished and too casual, so 요 is added at the end to keep it polite.