Because Connector: -서 vs -니까¶
Quick Reference
Pattern 1: Verb stem + 아서/어서/여서 Pattern 2: Verb stem + (으)니까 Meaning: "because, so, since" Key Difference: -서 = natural consequence, objective | -니까 = subjective reason, justification
Detailed Explanation¶
Both -서 and -니까 express cause and reason, but they have important nuanced differences in usage, formality, and the relationship between clauses.
Formation Rules¶
-아서/어서/여서¶
Follow vowel harmony rules:
- Stem ends in 아 or 오 → 아서
- 가다 → 가서
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좋다 → 좋아서
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Stem ends in other vowels → 어서
- 먹다 → 먹어서
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읽다 → 읽어서
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하다 verbs → 여서 (often contracts to 해서)
- 공부하다 → 공부하여서 → 공부해서
-(으)니까¶
Based on final consonant:
- Stem ends in vowel or ㄹ → 니까
- 가다 → 가니까
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만들다 → 만드니까 (ㄹ drops)
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Stem ends in consonant → 으니까
- 먹다 → 먹으니까
- 읽다 → 읽으니까
Key Differences Between -서 and -니까¶
1. Subjectivity Level¶
-서: Objective, natural consequence - States facts and obvious connections - The result follows naturally from the cause
-니까: Subjective reasoning, justification - Speaker's interpretation or reasoning - Often used when explaining or justifying
Examples: - 비가 와서 길이 젖었어요. It rained, so the street is wet. (natural consequence)
- 비가 오니까 우산을 가져가세요. Since it's raining, take an umbrella. (speaker's suggestion based on rain)
2. Commands and Suggestions¶
-서: CANNOT be used with commands, suggestions, or requests in the second clause
-니까: CAN be used with commands, suggestions, or requests
- ❌ 추워서 창문을 닫으세요.
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✓ 추우니까 창문을 닫으세요. It's cold, so please close the window.
-
❌ 늦어서 빨리 가세요.
- ✓ 늦으니까 빨리 가세요. We're late, so please go quickly.
3. Speaker's New Realization¶
-니까: Can express a new realization or discovery -서: Does not carry this meaning
- 집에 가니까 아무도 없었어요. When I went home, (I found/realized) no one was there.
4. Formality and Strength¶
-니까: Stronger, more direct, can sound formal or assertive -서: Softer, more natural in casual conversation
5. Question Form¶
-니까: Can be used in questions (asking for the listener's reason) -서: Cannot be used in questions
- 왜 학교에 안 가니까? Why aren't you going to school?
Usage Examples¶
Using -서¶
Objective Cause and Effect¶
-
배가 고파서 밥을 먹었어요. I was hungry, so I ate.
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너무 피곤해서 일찍 잤어요. I was very tired, so I slept early.
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시간이 없어서 택시를 타야 해요. I don't have time, so I have to take a taxi.
Natural Consequences¶
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열심히 공부해서 시험을 잘 봤어요. I studied hard, so I did well on the exam.
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어제 많이 걸어서 다리가 아파요. I walked a lot yesterday, so my legs hurt.
Using -니까¶
With Commands/Suggestions¶
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위험하니까 조심하세요. It's dangerous, so please be careful.
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맛있으니까 많이 드세요. It's delicious, so please eat a lot.
Justifications/Explanations¶
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제가 한국 사람이니까 한국어를 잘해요. Since I'm Korean, I speak Korean well.
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여기가 우리 집이니까 편하게 있으세요. Since this is our house, please be comfortable.
New Realizations¶
- 가방을 열어 보니까 지갑이 없었어요. When I opened my bag, (I found) my wallet wasn't there.
Comparison Chart¶
| Feature | -아서/어서 | -(으)니까 |
|---|---|---|
| Objectivity | Objective, factual | Subjective, interpretive |
| With commands | ✗ Cannot use | ✓ Can use |
| With suggestions | ✗ Cannot use | ✓ Can use |
| In questions | ✗ Cannot use | ✓ Can use |
| Formality | Neutral, softer | Can be formal/assertive |
| Realization | ✗ No | ✓ Yes |
| Natural consequence | ✓ Yes | ✓ Yes |
Practice¶
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Fill in the blank (choose -서 or -니까): 날씨가 좋___ 산책하러 갔어요.
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Which is correct for a command?: a) 위험해서 하지 마세요. b) 위험하니까 하지 마세요.
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Connect with -서: 어제 바빴다 + 친구를 못 만났어요.
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Connect with -니까: 비가 오다 + 우산을 가져가세요.
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Explain why this is wrong: 시간이 없어서 빨리 가세요.
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Translate using -서: I didn't have money, so I couldn't buy it. (돈이 없다, 사다)
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Translate using -니까: It's late, so please go home. (늦다, 집에 가다)
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Which should you use?: 문을 열다 + (I realized) 고양이가 있었어요.
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Create a sentence with -서 expressing natural consequence (비가 오다, 길이 미끄럽다)
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Create a sentence with -니까 giving a suggestion (피곤하다, 쉬다)