Comparing -아서/-어서 and -(으)니까

Both -아서/-어서 and -(으)니까 connect a reason to a result, but they have different tendencies in how they are used.

-아서/-어서: Natural Cause and Effect

This ending is widely used to state a simple, factual reason and its natural result.

시간이 없어서 택시를 탔어요. (I didn't have time, so I took a taxi.)

-(으)니까: Subjective Reason

This ending often provides a reason based on the speaker's judgment or realization, setting up a foundation for the next clause.

시간이 없으니까 빨리 출발합시다. (Since we don't have time, let's depart quickly.)

The most important boundary between these two connectors involves the end of the sentence.

Commands and Suggestions

-아서/-어서 cannot be used when the sentence ends with a command (Please do...) or a suggestion (Let's do...). For these situations, you typically use -(으)니까 .

Situation · Connected Sentence

Stating a fact · 피곤해서 집에 갔어요. (I was tired, so I went home.)

Making a request · 피곤하니까 먼저 집에 가세요. (Because you are tired, please go home first.)

Making a suggestion · 시간이 없으니까 빨리 출발합시다. (Since we don't have time, let's depart quickly.)

Common Mistake: Do not use -아서/-어서 with commands. "피곤해서 먼저 집에 가세요" is incorrect.

Another structural difference is how these connectors interact with tense markers like the past tense ( -았/-었 ).

-아서/-어서: Tense at the End

You cannot attach tense markers directly before -아서/-어서 . The time of the event is shown by the final verb in the sentence.

비가 와서 우산을 샀어요.

비가 왔어서 우산을 샀어요.

-(으)니까: Tense Allowed

You can attach past or future tense markers directly before -(으)니까 to emphasize when the reason occurred.

어제 아팠으니까 오늘은 쉬세요. (Since you were sick yesterday, please rest today.)

There is one specific situation where -아서/-어서 is almost always preferred over -(으)니까 .

Expressing Feelings

When you are giving the reason for an emotion, an apology, or gratitude, -아서/-어서 is the standard connector.

만나서 반갑습니다.

늦어서 죄송합니다.

도와주셔서 감사합니다.

Using -(으)니까 in these common greetings sounds unnatural in Korean.

Quick review

feelings and apologies

만나서 반갑습니다