Expressing Reasons and Causes: -아/어서, -(으)니까, and -기 때문에

Explaining why something happened or why someone should do something is a core part of everyday communication. In beginner Korean, you likely learned to state simple facts. As an intermediate learner, you need to connect those facts to show cause and effect.

Korean has several ways to say "because" or "so," but they are not interchangeable. Using the wrong connective ending can make a sentence sound unnatural or grammatically incorrect. This lesson breaks down the three most common reason connectives—how to build them, when to use them, and the specific rules that govern them.

Learning Objectives

• Connect sentences using -아/어서 (-a/eoseo) to express natural causes and sequences.
• Use -(으)니까 (-(eu)nikka) to provide reasons for commands and suggestions.
• Express strong, objective reasons using -기 때문에 (-gi ttaemune).
• Choose the correct connective based on the sentence ending (statements vs. commands).

Prerequisites

• Familiarity with basic verb and adjective conjugation (present and past tense).
• Understanding of formal/polite sentence endings like -아요/어요 (-ayo/eoyo) and commands like -(으)세요 (-(eu)seyo).

Core Concepts

1. The Natural Reason: -아/어서 (-a/eoseo)

This is the most common way to say "because" or "so" in everyday conversation. It is used to express a natural, logical cause and effect.

To conjugate, use the same rules as the standard -아요/어요 (-ayo/eoyo) present tense, but drop the 요 (yo) and add 서 (seo).

• 가다 (gada - to go) → 가서 (gaseo - because I go / so I go)
• 먹다 (meokda - to eat) → 먹어서 (meogeoseo - because I eat / so I eat)
• 피곤하다 (pigonhada - to be tired) → 피곤해서 (pigonhaeseo - because I am tired / so I am tired)

Examples:

• 배가 아파서 병원에 가요. (Baega apaseo byeongwone gayo.) — Because my stomach hurts, I am going to the hospital.
• 비가 와서 집에 있어요. (Biga waseo jibe isseoyo.) — It is raining, so I am staying home.

Crucial Rules for -아/어서 (-a/eoseo):

1. No past or future tense markers before it. You cannot attach -았/었- (-at/eot-) or -겠- (-get-) to the verb before -아/어서 (-a/eoseo). The tense is determined by the final verb of the sentence.
2. No commands or suggestions after it. You cannot end the sentence with -(으)세요 (-(eu)seyo - please do) or -(으)ㅂ시다 (-(eu)psida - let's do).

2. The Justification: -(으)니까 (-(eu)nikka)

This connective is used when the reason is subjective, a personal discovery, or a justification for an action.

To conjugate, attach -니까 (-nikka) to verb stems ending in a vowel, and -으니까 (-eunikka) to verb stems ending in a consonant.

• 오다 (oda - to come) → 오니까 (onikka - because it comes)
• 늦다 (neutda - to be late) → 늦으니까 (neujeunikka - because it is late)
• 춥다 (chupda - to be cold) → 추우니까 (chuunikka - because it is cold) Note: ㅂ irregular rule applies.

Examples:

• 날씨가 추우니까 따뜻한 옷을 입으세요. (Nalssiga chuunikka ttatteuthan oseul ibeuseyo.) — Because the weather is cold, please wear warm clothes.
• 시간이 없으니까 빨리 갑시다. (Sigani eopseunikka ppalli gapsida.) — Because we don't have time, let's go quickly.

Crucial Rules for -(으)니까 (-(eu)nikka):

1. Perfect for commands and suggestions. Unlike -아/어서 (-a/eoseo), this is the only correct choice when telling someone to do something or suggesting an action.
2. Can take tense markers. You can (and should) use past tense markers before -(으)니까 (-(eu)nikka) if the reason happened in the past.
   • 어제 바빴으니까 오늘 쉴게요. (Eoje bappasseunikka oneul swilgeyo.) — Because I was busy yesterday, I will rest today.

3. The Objective Cause: -기 때문에 (-gi ttaemune)

This connective expresses a very clear, strong, and objective reason. It translates closely to "due to" or "because of the fact that." It is frequently used in formal speech, writing, or when you want to emphasize the reason heavily.

To conjugate, simply attach -기 때문에 (-gi ttaemune) to the verb or adjective stem, regardless of vowels or consonants.

• 비싸다 (bissada - to be expensive) → 비싸기 때문에 (bissagi ttaemune - because it is expensive)
• 먹다 (meokda - to eat) → 먹기 때문에 (meokgi ttaemune - because I eat)

If you are using a noun, you drop the -기 (-gi) and just attach 때문에 (ttaemune).

• 일 (il - work) → 일 때문에 (il ttaemune - because of work)
• 비 (bi - rain) → 비 때문에 (bi ttaemune - because of the rain)

Examples:

• 외국인이기 때문에 한국어를 잘 못해요. (Oeguginigi ttaemune hangugeoreul jal mothaeyo.) — Because I am a foreigner, I cannot speak Korean well.
• 회의 때문에 바빠요. (Hoe-ui ttaemune bappayo.) — I am busy because of the meeting.

Crucial Rule for -기 때문에 (-gi ttaemune): Like -아/어서 (-a/eoseo), you cannot use commands or suggestions at the end of the sentence.

Common Mistakes

Mistake 1: Using -아/어서 (-a/eoseo) with a command.

• Incorrect: 비가 와서 우산을 가져가세요. (Biga waseo usaneul gajeogaseyo.)
• Why it happens: Learners translate "because" directly without considering the sentence ending.
• Correct: 비가 오니까 우산을 가져가세요. (Biga onikka usaneul gajeogaseyo.)
• Tip: If the sentence ends in "please do" or "let's," always use -(으)니까 (-(eu)nikka).

Mistake 2: Putting past tense before -아/어서 (-a/eoseo).

• Incorrect: 어제 바빴어서 친구를 못 만났어요. (Eoje bappasseoseo chingureul mot mannasseoyo.)
• Why it happens: Learners want to show that the reason happened in the past.
• Correct: 어제 바빠서 친구를 못 만났어요. (Eoje bappaseo chingureul mot mannasseoyo.)
• Tip: -아/어서 (-a/eoseo) never takes past tense. The final verb (만났어요) already shows the sentence is in the past.

Practice Prompts

1. Think of a reason why you were late to an event recently. How would you explain it using -아/어서 (-a/eoseo)?
2. Imagine it is raining outside. How would you tell a friend to take an umbrella using -(으)니까 (-(eu)nikka)?
3. Think of a noun that causes you stress (e.g., work, exams). Create a sentence using Noun + 때문에 (ttaemune) to explain why you are tired.

Key Takeaways

• Use -아/어서 (-a/eoseo) for general, natural reasons. Never use it with past tense markers or commands.
• Use -(으)니까 (-(eu)nikka) when giving a reason for a command (-(으)세요) or suggestion (-(으)ㅂ시다). It can take past tense markers.
• Use -기 때문에 (-gi ttaemune) for strong, objective, or formal reasons. Use Noun + 때문에 (ttaemune) for "because of [noun]."

Vocabulary List

Verbs & Adjectives

• 가다 (gada) — to go
• 먹다 (meokda) — to eat
• 오다 (oda) — to come
• 늦다 (neutda) — to be late
• 입다 (ipda) — to wear
• 가져가다 (gajeogada) — to take, to bring
• 만나다 (mannada) — to meet
• 쉬다 (swida) — to rest
• 아프다 (apeuda) — to be sick, to hurt
• 피곤하다 (pigonhada) — to be tired
• 바쁘다 (bappeuda) — to be busy
• 춥다 (chupda) — to be cold
• 비싸다 (bissada) — to be expensive

Nouns

• 배 (bae) — stomach
• 병원 (byeongwon) — hospital
• 비 (bi) — rain
• 집 (jip) — house, home
• 날씨 (nalssi) — weather
• 옷 (ot) — clothes
• 시간 (sigan) — time
• 우산 (usan) — umbrella
• 친구 (chingu) — friend
• 일 (il) — work
• 회의 (hoe-ui) — meeting
• 외국인 (oegugin) — foreigner
• 한국어 (hangugeo) — Korean language

Phrases & Endings

• -(으)세요 (-(eu)seyo) — please do (polite command)
• -(으)ㅂ시다 (-(eu)psida) — let's do (suggestion)
• 빨리 (ppalli) — quickly, fast
• 잘 못해요 (jal mothaeyo) — cannot do well