Both -아/어서 and -(으)니까 express causality ("because," "since"), but they have important differences in usage, emphasis, and grammatical restrictions. Understanding when to use each is crucial for natural Korean.
Overview
-아/어서
- Neutral, softer reason
- Natural sequence
- Cannot use with commands/suggestions
- Cannot use past tense
- More conversational
-(으)니까
- Emphasized reason
- Strong justification
- Can use with commands/suggestions
- Can use past tense
- More direct, assertive
Major Difference 1: Commands and Suggestions
-아/어서: Cannot Use
With commands (imperative): ❌ 추워서 코트를 입으세요. (incorrect) ❌ 위험해서 조심하세요. (incorrect)
With suggestions (propositive): ❌ 배고파서 밥을 먹읍시다. (incorrect) ❌ 시간 있어서 갑시다. (incorrect)
-(으)니까: Can Use
With commands (imperative): ✓ 춥으니까 코트를 입으세요. (correct) ✓ 위험하니까 조심하세요. (correct)
With suggestions (propositive): ✓ 배고프니까 밥을 먹읍시다. (correct) ✓ 시간 있으니까 갑시다. (correct)
Explanation
-아/어서 describes natural sequences and cannot impose actions on others. -(으)니까 provides justification for commands or suggestions.
Major Difference 2: Past Tense
-아/어서: Cannot Use Past Tense
The first clause cannot take -았/었:
❌ 비가 왔어서 못 갔어요. (incorrect) ✓ 비가 와서 못 갔어요. (correct)
❌ 공부했어서 잘 봤어요. (incorrect) ✓ 공부해서 잘 봤어요. (correct)
-(으)니까: Can Use Past Tense
Can add -았/었 to show past reason:
✓ 비가 왔으니까 못 갔어요. (correct) ✓ 공부했으니까 잘 볼 거예요. (correct) ✓ 먹었으니까 배가 안 고파요. (correct)
When This Matters
When you need to emphasize a past action as the reason:
Cannot use -아/어서:
- Must use: 먹어서 배가 안 고파요. (ate, so not hungry)
Can use -(으)니까:
- Better: 먹었으니까 배가 안 고파요. (because I ate, not hungry)
Difference 3: Emphasis and Tone
-아/어서: Neutral, Softer
Natural, conversational:
-
피곤해서 쉬고 싶어요. (I'm tired, so I want to rest.) - neutral statement
-
비싸서 안 샀어요. (It was expensive, so I didn't buy it.) - matter-of-fact
Feels like:
- Natural flow of events
- Objective observation
- Softer, less assertive
-(으)니까: Emphasized, Stronger
More assertive:
-
피곤하니까 쉬고 싶어요. (Since I'm tired, I want to rest.) - stronger reason
-
비싸니까 사지 마세요. (Since it's expensive, don't buy it.) - stronger advice
Feels like:
- Emphasizing the reason
- Justifying or explaining
- More direct, definite
Difference 4: Usage Context
-아/어서: Better For
1. Simple cause-effect statements:
- 배가 고파서 밥을 먹었어요. (I was hungry, so I ate.)
2. Sequential actions:
- 집에 가서 쉬었어요. (I went home and rested.)
3. Thanking and apologizing:
-
도와주셨어서 감사합니다. (Thank you for helping.)
-
늦어서 죄송합니다. (Sorry for being late.)
4. Natural, gentle explanations:
- 시간이 없어서 못 갔어요. (I didn't have time, so I couldn't go.)
-(으)니까: Better For
1. Commands with justification:
- 위험하니까 조심하세요. (Since it's dangerous, be careful.)
2. Suggestions:
- 시간 있으니까 천천히 합시다. (Since we have time, let's do it slowly.)
3. Strong justifications:
- 약속했으니까 지켜야 해요. (Since I promised, I must keep it.)
4. Discoveries after checking:
- 해 보니까 어려웠어요. (When I tried, it was difficult.)
5. Warnings:
- 미끄럽으니까 조심하세요. (Since it's slippery, be careful.)
Comparison in Similar Sentences
Example 1: Weather
With -아/어서 (neutral statement):
- 비가 와서 못 갔어요. (It rained, so I couldn't go.) - reporting what happened
With -(으)니까 (command/suggestion):
- 비가 오니까 우산을 가져가세요. (Since it's raining, take an umbrella.) - giving advice
Example 2: Being Busy
With -아/어서 (gentle refusal):
- 바빠서 못 가요. (I'm busy, so I can't go.) - softer explanation
With -(으)니까 (stronger reason):
- 바쁘니까 다음에 만나요. (Since I'm busy, let's meet next time.) - more definite
Example 3: Food Quality
With -아/어서 (reporting):
- 맛있어서 많이 먹었어요. (It was delicious, so I ate a lot.) - statement
With -(으)니까 (recommendation):
- 맛있으니까 꼭 드세요. (Since it's delicious, definitely try it.) - stronger suggestion
When Both Are Possible
Statements Without Commands
In simple statements, both can be used with slight nuance:
-아/어서 (softer):
- 좋아서 샀어요. (It was good, so I bought it.) - neutral
-(으)니까 (emphasized):
- 좋으니까 샀어요. (Since it was good, I bought it.) - emphasizes the reason
With Different Endings
In final statements (not commands), the choice affects tone:
Softer, more conversational:
- 피곤해서 일찍 잤어요.
More emphatic:
- 피곤하니까 일찍 잤어요.
Cannot Substitute: Commands
Must Use -(으)니까
When giving commands or suggestions, ONLY -(으)니까 works:
Commands: ✗ 추워서 입으세요. ✓ 춥으니까 입으세요.
✗ 늦어서 빨리 가세요. ✓ 늦었으니까 빨리 가세요.
Suggestions: ✗ 배고파서 먹읍시다. ✓ 배고프니까 먹읍시다.
Cannot Substitute: Past Emphasis
Must Use -(으)니까
When emphasizing a completed past action as the reason:
Want past tense emphasis: ✗ 공부했어서 잘 볼 거예요. (cannot add past) ✓ 공부해서 잘 볼 거예요. (can only do this) ✓ 공부했으니까 잘 볼 거예요. (past emphasis possible)
Politeness Comparison
Both Are Polite
Neither is inherently more or less polite:
-아/어서 (gentle, indirect):
- 바빠서 못 가요. (I'm busy, so I can't go.) - gentle refusal
-(으)니까 (direct, clear):
- 바쁘니까 못 가요. (Since I'm busy, I can't go.) - clearer reason
Context Determines Politeness
The final verb ending determines overall politeness level:
Informal polite (both):
- 피곤해서 쉬고 싶어요.
- 피곤하니까 쉬고 싶어요.
Formal polite (both):
- 피곤해서 쉬고 싶습니다.
- 피곤하니까 쉬고 싶습니다.
Common Patterns Comparison
Apologizing
Prefer -아/어서:
- 늦어서 죄송합니다. (Sorry for being late.) - more common
- 늦었으니까 죄송합니다. (less common)
Thanking
Prefer -아/어서:
- 도와주셔서 감사합니다. (Thank you for helping.) - more common
- 도와주셨으니까 감사합니다. (less common)
Giving Advice
Prefer -(으)니까:
- 위험하니까 조심하세요. (Be careful since it's dangerous.) - natural
- 위험해서 조심하세요. (incorrect - cannot use with command)
Making Suggestions
Must use -(으)니까:
- 시간 있으니까 갑시다. (Let's go since we have time.) - only option
- 시간 있어서 갑시다. (incorrect)
Regional and Stylistic Variations
Standard Usage
Both are standard and widely used.
Conversational Preference
In casual conversation, -아/어서 is often preferred for simple statements:
More casual:
- 피곤해서 못 가요.
More direct:
- 피곤하니까 못 가요.
Formal Writing
Both are acceptable in formal writing, but -(으)니까 may sound more definite:
Academic/business: Both acceptable, choose based on intended emphasis.
Quick Decision Guide
Use -아/어서 When:
- Making simple statements ✓
- Apologizing/thanking ✓
- Natural sequences ✓
- Softer explanations ✓
- NOT giving commands ✓
Use -(으)니까 When:
- Giving commands ✓ (MUST)
- Making suggestions ✓ (MUST)
- Need past tense ✓ (MUST)
- Emphasizing reasons ✓
- Justifying actions ✓
Common Mistakes
❌ Using -아/어서 with Commands
Wrong: 춥어서 입으세요. Right: 춥으니까 입으세요.
❌ Adding Past to -아/어서
Wrong: 먹었어서 배불러요. Right: 먹어서 배불러요. Right: 먹었으니까 배불러요. (if want past emphasis)
❌ Overusing -(으)니까
While -(으)니까 is versatile, don't overuse it where -아/어서 sounds more natural:
Less natural: 피곤하니까 잤어요. More natural: 피곤해서 잤어요.
Summary Table
| Feature | -아/어서 | -(으)니까 |
|---|---|---|
| Commands | ✗ Cannot | ✓ Can |
| Suggestions | ✗ Cannot | ✓ Can |
| Past tense | ✗ Cannot | ✓ Can |
| Emphasis | Neutral | Strong |
| Tone | Softer | More direct |
| Apologizing | ✓ Preferred | Less common |
| Thanking | ✓ Preferred | Less common |
| Giving advice | ✗ Cannot (command) | ✓ Common |