Because / And Then (-아/어서)

-아/어서 has two main uses: expressing reason/cause ('because') and connecting naturally sequential actions. The form follows vowel harmony: use -아서 after stems with ㅏ or ㅗ, and -어서 after all other vowels. 하다 becomes 해서.

For causal use, 배가 아파서 병원에 갔어요 means 'Because my stomach hurt, I went to the hospital.' For sequential use, 서울에 가서 친구를 만났어요 means 'I went to Seoul and met a friend.' The sequential use implies the actions are closely connected or that one naturally leads to the other.

A key restriction is that -아/어서 cannot be used with imperative or propositive endings. You cannot say *배가 아파서 병원에 가세요. For commands and suggestions with a reason, use -(으)니까 instead. Additionally, the tense marker is placed only on the final verb, not on -아/어서.