Because / So

This pattern connects two clauses to show a cause-and-effect relationship. The first clause, ending in -아/어서, gives the reason or cause for the action or state described in the second clause. It's equivalent to saying "because..." or "so..." in English.

Formation

For verbs/adjectives with a stem ending in ㅏ or ㅗ: Add -아서.

가다 (to go) → 가서

보다 (to see) → 봐서 (irregular contraction)

만나다 (to meet) → 만나서

For verbs/adjectives with a stem ending in other vowels: Add -어서.

먹다 (to eat) → 먹어서

읽다 (to read) → 읽어서

예쁘다 (to be pretty) → 예뻐서 (irregular contraction)

For 하다 verbs/adjectives: They become -해서.

피곤하다 (to be tired) → 피곤해서

For nouns: Add -(이)라서.

처음 (first time) → 처음이라서

학생 (student) → 학생이라서

Tense

The tense of the entire sentence is always indicated in the final verb of the second clause. The -아/어서 clause itself does not get conjugated for tense.

Correct: 배고파서 밥 먹었어요. (I was hungry, so I ate.)

Incorrect: 배고팠어서 밥 먹었어요.

배고파서 현기증 나. / 너무 비싸서 못 샀어. / 비가 와서 못 갔어요. / 처음이라서 긴장돼.

-아/어서 is a key component of many fixed, high-frequency expressions in Korean for greetings, apologies, and thanks. In these cases, the cause-and-effect logic is still there, but they function like set phrases.

Greetings: 만나서 반갑습니다. (Nice to meet you.)

Reason: We met. Result: I am pleased.

Apologies: 늦어서 미안해요. (Sorry I'm late.)

Reason: I am late. Result: I am sorry.

Gratitude: 도와줘서 고마워요. (Thank you for helping.)

Reason: You helped me. Result: I am thankful.

Memorizing these common phrases will help you sound more natural and use -아/어서 instinctively.

만나서 반가워. / 늦어서 미안해요. / 도와줘서 고마워요.

A common point of confusion for learners is the difference between -아/어서 and -(으)니까, as both can mean "because."

The most important rule is that -아/어서 cannot be followed by commands or suggestions. It is used to state a simple, objective cause and effect.

-(으)니까, on the other hand, is often used to provide a reason to justify a following command, suggestion, or proposal.

Usage Rules:

Stating a fact/result: Use -아/어서.

더워서 에어컨 켰어요. (It was hot, so I turned on the AC.) - Correct

Making a suggestion/command: Use -(으)니까.

더우니까 에어컨 켤까요? (Since it's hot, shall we turn on the AC?) - Correct

더워서 에어컨 켤까요? - Incorrect & Unnatural

Think of -아/어서 for neutral explanations and -(으)니까 when you're trying to lead the listener to a specific action.

길이 막혀서 지하철 탔어요. (O) / 길이 막히니까 지하철 탑시다. (O) / 길이 막혀서 지하철 탑시다. (X)

The -아/어서 pattern itself does not change for politeness. The formality of your sentence is determined entirely by the ending of the final verb.

As you can see in the script lines, the same reason can be expressed casually to a friend or politely to someone older or less familiar.

Casual (반말): Ends with forms like -어, -았어, etc.

길이 막혀서 늦었어. (I was late because of traffic.)

Polite (해요체): Ends with -아요/어요.

길이 막혀서 늦었어요. (I was late because of traffic.)

This flexibility allows you to use the pattern in almost any daily situation, simply by adjusting the final verb ending.

네가 보고 싶어서 왔어. / 호리 씨가 보고 싶어서 왔어요. / 더워서 에어컨 켰어. / 더워서 에어컨 켰어요.