Korean Grammar -아서/어서 (So, Because, Then) 

 August 4, 2025

By  Dr. Satish Satyarthi

Korean Grammar: -아서/어서 (Because / So / And Then)

Today, we will learn a fundamental and widely-used Korean grammar pattern: -아서/어서.

You will encounter this pattern frequently in both spoken and written Korean. It acts as a connector, joining two clauses (parts of a sentence). It has two main functions:

  1. To indicate a reason or cause (similar to “because” or “so”).
  2. To list two actions that happen sequentially (like “and then”).

Let’s explore how to use this versatile pattern.

1. Usage: Understanding -아서/어서

This grammar pattern connects two verbs or adjectives, creating a logical link between them.

Function 1: Reason / Cause (“because” or “so”)

This is its most common function. The first clause (containing -아서/어서) states the reason or cause, and the second clause states the result.

  • Korean: 피곤해서 쉬고 있어요.
    English: “I am tired, so I am resting.” (Reason: I am tired → Result: I am resting)
  • Korean: 비가 와서 집에 있어요.
    English: “Because it’s raining, I’m staying home.” (Reason: It’s raining → Result: I’m staying home)

Function 2: Sequential Actions (“and then”)

This function connects two actions that happen in order. The action in the first clause must be completed before the second action begins. The two actions are closely related.

  • Korean: 공원에 가서 운동해요.
    English: “I go to the park and then I exercise.” (Action 1: Go to the park → Action 2: Exercise)
  • Korean: 친구를 만나서 영화를 봤어요.
    English: “I met a friend and then we watched a movie.” (Action 1: Meet friend → Action 2: Watch movie)

2. Conjugation: How to Form -아서/어서

To use this pattern, you attach -아서 or -어서 to the stem of a verb or adjective (the part you get after dropping -다).

Rule 1: Stems with ㅏ or ㅗ

If the last vowel of the verb/adjective stem is ㅏ or ㅗ, you add -아서.

  • 가다 (to go) → 가 + -아서 = 가서
  • 보다 (to see/watch) → 보 + -아서 = 봐서 (ㅗ + ㅏ combine to become ᅪ)
  • 좋다 (to be good) → 좋 + -아서 = 좋아서

Rule 2: Stems with other vowels

If the last vowel of the stem is anything other than ㅏ or ㅗ (like ㅓ, ㅜ, ㅡ, ㅣ), you add -어서.

  • 먹다 (to eat) → 먹 + -어서 = 먹어서
  • 읽다 (to read) → 읽 + -어서 = 읽어서
  • 마시다 (to drink) → 마시 + -어서 = 마셔서 (ㅣ + ㅓ combine to become ㅕ)
  • 예쁘다 (to be pretty) → 예쁘 + -어서 = 예뻐서 (ㅡ is dropped and ㅓ is added)

Rule 3: 하다 Verbs

All verbs ending in 하다 (like 공부하다, 일하다) become -해서.

  • 공부하다 (to study) → 공부해서
  • 일하다 (to work) → 일해서
  • 요리하다 (to cook) → 요리해서

Tense: A Very Important Rule

This is a critical rule for -아서/어서: You cannot add past tense markers (-았/었-) to the verb before -아서/어서. The tense of the entire sentence (past, present, or future) is only expressed in the final verb at the end.

Correct: 머리가 아파서 학교에 안 갔어요. (My head hurt, so I didn’t go to school.)
Incorrect: ❌ 머리가 아팠어서 학교에 안 갔어요. ❌
Correct: 피곤해서 일찍 잘 거예요. (I’m tired, so I will sleep early.)
Incorrect: ❌ 피곤할 거라서 일찍 잘 거예요. ❌

Using -아서/어서 with Nouns

When the reason is a noun, you use (이)라서.

  • Noun ends in a consonant: + 이라서
    Example: 학생 (student) → 학생이라서 (because I’m a student)
  • Noun ends in a vowel: + 라서
    Example: 의사 (doctor) → 의사라서 (because I’m a doctor)

3. Example Sentences with -아서/어서

Here are simple examples showing -아서/어서 in different situations.

(Function 1: Because / So)

  1. 날씨가 좋아서 기분이 좋아요.
    The weather is nice, so I feel good.
  2. 배가 고파서 빵을 먹었어요. (From 고프다)
    I was hungry, so I ate bread.
  3. 바빠서 전화를 못 받았어요. (From 바쁘다)
    I was busy, so I couldn’t answer the phone.
  4. 한국어가 재미있어서 매일 공부해요.
    Korean is fun, so I study every day.
  5. 주말이라서 백화점에 사람이 많아요. (Noun: 주말)
    Because it’s the weekend, there are many people at the department store.

(Function 2: And Then)

  1. 집에 가서 저녁을 먹을 거예요.
    I will go home and then eat dinner.
  2. 친구를 만나서 커피를 마셨어요.
    I met my friend and then we drank coffee.
  3. 책을 사서 읽었어요.
    I bought a book and then read it.
  4. 아침에 일어나서 물을 마셔요.
    I wake up in the morning and then drink water.

4. Key Rules and Comparisons

Here are a few crucial rules and comparisons to master -아서/어서.

Rule 1: No Commands or Suggestions with “Because”

When using -아서/어서 to express a reason, you CANNOT end the sentence with a command (e.g., -(으)세요) or a suggestion (e.g., -(으)ㅂ시다, -(으)ㄹ까요?).

For commands and suggestions, you must use the ‘because’ grammar -(으)니까 instead.

Incorrect: ❌ 날씨가 좋아서 공원에 갈까요?
Correct: 날씨가 좋으니까 공원에 갈까요? (Since the weather is nice, shall we go to the park?)
Incorrect: ❌ 바빠서 나중에 전화하세요.
Correct: 바쁘니까 나중에 전화하세요. (Since I am busy, please call later.)

Rule 2: Tense Reminder

As mentioned in the conjugation section, this is very important. The tense is only marked on the final verb of the sentence. Do not add past tense markers like -았/었- before -아서/어서.

Rule 3: Notes on Sequential Use

When using -아서/어서 for sequential actions (“and then”), two conditions must be met:

  1. The two actions must be closely related.
  2. The subject (the person performing the actions) must be the same for both clauses.
Correct: 집에 가서 쉬어요. [I] go home and [I] rest.
Incorrect: 제가 집에 가고 아버지가 티비를 봐요. ❌ [I] go home and [my father] watches TV. (The subjects, “I” and “the sun,” are different).
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Dr. Satish Satyarthi

Dr. Satish is the founder of LKI School of Korean Language. He got his Masters degree in Korean Language Education from Seoul National University, South Korea under GKS scholarship. He did his M.Phil. and PhD in Korean Language from Jawaharlal Nehru University, Delhi. He also taught at Centre for Korean Studies, JNU for 2 years. He has a passion for education and technology. He has been involved in Korean language education and research for more than 10 years and has trained thousands of Korean language learners across the globe both online and in physical classrooms.

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