-아서/어서 vs -(으)니까: Knowing When to Use Each
Both mean because but they are not interchangeable. One simple rule tells you which to use every time.
Both -아서/어서 and -(으)니까 mean "because" or "so," but mixing them up is one of the most common mistakes Korean learners make. Here's the rule that clears it up instantly.
The Golden Rule
-아서/어서 cannot be followed by commands, requests, or suggestions.
-(으)니까 can be followed by anything.
That one rule eliminates most errors.
Side-by-Side Examples
| Situation | Wrong | Right |
|---|---|---|
| Command | 피곤해서 쉬세요 ❌ | 피곤하니까 쉬세요 ✅ |
| Suggestion | 날씨가 좋아서 나갈까요? ❌ | 날씨가 좋으니까 나갈까요? ✅ |
| Request | 맛있어서 더 주세요 ❌ | 맛있으니까 더 주세요 ✅ |
When -아서/어서 Works Best
Use -아서/어서 for natural cause-and-effect when describing what happened:
- 배가 고파서 밥을 먹었어요. (I was hungry, so I ate.)
- 비가 와서 집에 있었어요. (It rained, so I stayed home.)
- 늦어서 미안해요. (I'm sorry for being late.)
Note: -아서/어서 cannot attach to past tense — drop 었/았 and use the verb stem.
When -(으)니까 Works Best
Use -(으)니까 when giving a reason for a command, request, or suggestion:
- 배고프니까 빨리 먹자. (I'm hungry, let's eat quickly.)
- 내일 시험이 있으니까 오늘 공부해. (There's an exam tomorrow, so study today.)
Diary Writing Tip
When journaling in Korean, ask yourself: what comes after the reason?
- Describing what happened → -아서/어서
- Giving a reason for a plan or suggestion → -(으)니까
Try this in your diary tonight: 오늘 날씨가 좋으니까 산책하러 갔어요. (The weather was nice today, so I went for a walk.)
Practice the pattern tonight. Write that diary sentence and get instant AI corrections on grammar and naturalness → Korean Diary AI