This is the most tested grammar rule in beginner Korean:
When the second clause is a command (*-(으)세요*) or a suggestion (*-(으)ㅂ시다 / -(으)ㄹ까요?*):
❌ You CANNOT use -아/어서! ⭕ You MUST use -(으)니까!
Because it rains, take an umbrella비가 와서 우산 쓰세요 ❌ · 비가 오니까 우산 쓰세요 ⭕
Because it is hot, let's turn on A/C더워서 에어컨 켭시다 ❌ · 더우니까 에어컨 켭시다 ⭕
Because weather is nice, shall we go?날씨가 좋아서 갈까요? ❌ · 날씨가 좋으니까 갈까요? ⭕
Formal Written Reasons (-기 때문에): In formal speeches or writing, you also see -기 때문에 (for nouns: 때문에):
- 비 때문에 늦었어요. *(Late because of rain.)* - 비가 오기 때문에 길이 막혀요. *(Traffic is heavy because it rains.)* Like -아/어서, it is also not used with commands or suggestions.
연습 · Practice
이 레슨에는 풀어 보는 문제가 6개 있습니다. 앱에서 하나씩 맞히며 넘어갑니다. 6 exercises come with this lesson — open it to work through them.