-아/어서 vs -(으)니까: How to Say “Because” in Korean (3 Easy Rules)

Korean has two common ways to say “because”: -아/어서 and -(으)니까. Most textbooks translate both as “because” or “so,” which makes them look the same.

Level: Beginner · TOPIK 1-2

They are not the same. In some sentences only one of them is correct. In others, both are correct but sound different. This post covers both cases.

The Short Answer: -아/어서 vs -(으)니까

-아/어서 states a cause as a simple fact.
-(으)니까 gives a reason the speaker is using to make a point, ask for something, or suggest something.

Here is the same idea with each ending.

Let’s break down these two sentences.

Part	Meaning
비	rain
가	subject marker
오다 → 와서	to come + -아/어서 (because)
집에 있었어요	stayed at home

Part	Meaning
오다 → 오니까	to come + -(으)니까 (because)
우산을	umbrella + object marker
가져가세요	please take (it with you)

In Korean, rain “comes” (비가 오다). The first sentence just reports what happened. The second uses the rain as a reason to tell someone what to do.

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How to Form Them

Verb / adjective	-아/어서	-(으)니까
가다 (to go)	가서 gaseo	가니까 ganikka
오다 (to come)	와서 waseo	오니까 onikka
먹다 (to eat)	먹어서 meogeoseo	먹으니까 meogeunikka
공부하다 (to study)	공부해서 gongbuhaeseo	공부하니까 gongbuhanikka
덥다 (to be hot)	더워서 deowoseo	더우니까 deounikka

• -아서: when the last vowel of the stem is ㅏ or ㅗ
• -어서: all other vowels
• 해서: verbs ending in 하다
• -으니까: when the stem ends in a consonant
• -니까: when the stem ends in a vowel

Verbs like 덥다 change their form before both endings. These are called ㅂ-irregular verbs.

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Three Rules That Decide for You

Before thinking about nuance, check these three rules. They cover most of the cases where only one ending is correct.

Rule 1: Commands and suggestions need -(으)니까

If the second half of the sentence tells someone to do something (-(으)세요) or says “let’s” (-자), use -(으)니까. -아/어서 does not work here.

Native tip: “Shall we…?” questions
When Koreans suggest something with -(으)ㄹ까요? (“Shall we…?”), they usually give the reason with -는데, not -(으)니까.

비도 오는데 택시 타고 갈까요?
bido oneunde taeksi tago galkkayo?
It’s raining, so shall we take a taxi?

날씨도 좋은데 산책할까요?
nalssido joeunde sanchaekhalkkayo?
The weather’s nice, so shall we go for a walk?

비가 오니까 택시 타고 갈까요? is grammatically correct, but it sounds stiff. Notice the 도 (“also”) in these sentences. It makes the suggestion sound softer and more natural.

🗣️ Native insight Why does 비가 오니까 택시 탈까요? sound stiff to Koreans? The one small ending natives use instead, and when -(으)니까 is still right. Read the answer →

Rule 2: -아/어서 cannot take the past tense

With -아/어서, the tense goes at the end of the sentence, not before the ending. -(으)니까 can take the past tense directly.

Rule 3: Apologies and thanks use -아/어서

When you say sorry or thank you, use -아/어서. With -(으)니까, the reason sounds like an excuse or an argument.

The same goes for greetings like 만나서 반가워요 (mannaseo bangawoyo, “Nice to meet you”).

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When You’re Just Explaining

When you simply explain why something happened, -아/어서 is the natural choice. -(으)니까 can sound a little off in the same sentence.

Koreans who want to use -(으)니까 here usually end the sentence with -죠 instead.

This version is natural, but it has an attitude. It sounds like “Isn’t it obvious?” and can come across as annoyed or even rude. When you turn down an invitation politely, use -아/어서.

Native Speaker’s Note

In everyday conversation, Koreans often end a sentence with -(으)니까요, like 바쁘니까요 (“Because I’m busy”). It works as a short answer to “Why?”

The tone depends on how you say it. Said softly, it is a normal explanation. Said firmly, it can sound like “Obviously, because I’m busy.” In casual speech, you will also hear it pronounced as -니깐.

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Two Look-Alike Meanings

Both endings have a second meaning that has nothing to do with “because.” Learners often mix these up.

-아/어서: “and then”

With movement verbs like 가다 and 만나다, -아/어서 can link two actions in order.

Meeting the friend is not the reason for the movie. It is the first step.

-(으)니까: “when I did …, I found …”

When the second half is something you discovered, -(으)니까 means “when I did this, I found that.”

Going outside did not cause the snow. It is how the speaker noticed it.

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Summary and Quick Quiz

	-아/어서	-(으)니까
Basic meaning	Cause as a fact	Reason to make a point
Command / suggestion after it	❌	✅
Past tense before it	❌	✅
Sorry / thank you	✅	❌
Second meaning	“and then”	“when I did …, I found …”

Common mistakes

❌ Wrong	✅ Right	Why
비가 와서 우산을 가져가세요.	비가 오니까 우산을 가져가세요. biga onikka usaneul gajyeogaseyo.	The sentence ends with a request.
잠을 못 잤어서 피곤해요.	잠을 못 자서 피곤해요. jameul mot jaseo pigonhaeyo. I couldn’t sleep, so I’m tired.	No past tense before -아/어서.
와 줬으니까 고마워요.	와 줘서 고마워요. wa jwoseo gomawoyo. Thank you for coming.	Thanks use -아/어서.

What about -기 때문에?

-기 때문에 is a third way to say “because.” It sounds more formal and is common in writing and speeches. Like -아/어서, it is not used before commands or suggestions. We will cover it in a separate post.