Reason with -아/어서 in Korean

In Brief

Attach -아/어서 to a verb or descriptive verb stem to connect a cause to its result: 바빠서 못 가요 means “I can’t go because I’m busy.” Use -아서 after stems whose last vowel is ㅏ or ㅗ, -어서 after other vowels, and 해서 with 하다. The result normally cannot be a command or a “let’s” suggestion; use -(으)니까 in those cases.

Overview

Korean often joins a reason and its result in one sentence rather than placing a separate word equivalent to English “because” between two complete clauses. -아/어서 is a connective ending (an ending that links two sentence-like parts). The part before it presents the cause, reason, or background; the part after it presents what follows from that cause.

This is a core A2 pattern, but its building blocks should already feel familiar. It uses the same vowel choice as the polite -아/어요 ending: if you can form 가요, 먹어요, and 해요, you are close to forming 가서, 먹어서, and 해서. Only the final clause carries the sentence’s politeness ending, so 바빠서 못 가요 is polite even though 바빠서 itself has no -요.

English allows either order: “Because I was tired, I went home” or “I went home because I was tired.” With this Korean construction, keep the cause first: 피곤해서 집에 갔어요. This cause-first order is the safest and most natural model for learners.

How It Works

The basic pattern

The sentence has two clauses (parts that each express an action or state):

cause + -아/어서 + result

In 비가 와서 집에 있었어요 (“I stayed home because it rained”), 비가 와서 is the cause and 집에 있었어요 is the result. Do not add -요 to the connective form: not 와요서, but 와서.

Choosing -아서, -어서, or 해서

Remove -다 from the dictionary form to find the stem, then inspect the stem’s last vowel.

Stem type	Ending	Dictionary form	Connected form	Meaning
Last vowel ㅏ or ㅗ	-아서	작다	작아서	because it is small
Other final vowel	-어서	먹다	먹어서	because someone eats/ate
하다	해서	공부하다	공부해서	because someone studies/studied

The sound changes and contractions are the same ones seen with -아/어요.

Dictionary form	Formation	Result
가다 (go)	가 + 아서	가서
오다 (come)	오 + 아서	와서
보다 (see/watch)	보 + 아서	봐서
사다 (buy)	사 + 아서	사서
먹다 (eat)	먹 + 어서	먹어서
배우다 (learn)	배우 + 어서	배워서
마시다 (drink)	마시 + 어서	마셔서
쓰다 (write/use)	쓰 + 어서	써서
하다 (do)	하 + 여서	해서
되다 (become/work out)	되 + 어서	돼서

Common irregular patterns also follow the corresponding -아/어요 behavior:

Dictionary form	Connected form	Example meaning
덥다 (be hot)	더워서	because it is hot
춥다 (be cold)	추워서	because it is cold
모르다 (not know)	몰라서	because someone does not know
듣다 (listen/hear)	들어서	because someone heard/listened
낫다 (get better/be better)	나아서	because it is better / after recovering

Action verbs and descriptive verbs

The pattern attaches both to action verbs such as 먹다 (“eat”) and to descriptive verbs such as 바쁘다 (“be busy”). Korean descriptive verbs express qualities as predicates, where English usually needs “be + adjective.” That is why 바빠서 is one connected verb form rather than a separate word for “because” plus an adjective.

• 길이 막혀서 늦었어요. — I was late because traffic was bad.
• 방이 조용해서 공부하기 좋아요. — The room is quiet, so it is good for studying.

The two clauses often share an unstated subject, but they do not have to. Korean regularly leaves out a subject when context makes it clear.

• 제가 피곤해서 일찍 잤어요. — I was tired, so I went to bed early.
• 비가 와서 경기가 취소됐어요. — It rained, so the game was canceled.

Nouns with 이다 and 아니다

With the copula 이다 (the form that links a noun to “be”), common forms include -이어서/-여서 and -(이)라서. The -(이)라서 pattern is especially common in conversation.

Noun type	Form	Example
Ends in a consonant	-이어서 or -이라서	학생이어서 / 학생이라서
Ends in a vowel	-여서 or -라서	의사여서 / 의사라서
아니다	아니라서	학생이 아니라서

• 주말이라서 사람이 많아요. — There are many people because it is the weekend.
• 전문가가 아니라서 잘 모르겠어요. — I am not an expert, so I am not sure.

For an A2 learner, recognizing both sets is enough; using -(이)라서 consistently with nouns is a practical conversational choice.

Tense and politeness belong mainly to the final clause

In the beginner pattern, do not put a past or future marker before -아/어서. Let the final verb and context locate the whole event in time.

• 어제 아파서 학교에 못 갔어요. — I could not go to school yesterday because I was sick.
• 내일 바빠서 못 만날 거예요. — I will not be able to meet tomorrow because I will be busy.

Notice 아파서, not 아팠어서, and 바빠서, not a future-marked form. The final forms 갔어요 and 만날 거예요 establish the time. Only the final clause normally carries speech level and politeness: 죄송합니다, 죄송해요, or a casual ending can all follow the same connective clause.

The important sentence-type restriction

Use -아/어서 when the second clause states a result, reaction, or natural consequence. In the standard learner rule, do not use it when the second clause is:

• an imperative (a command or request), such as “take an umbrella”; or
• a propositive (a suggestion including the speaker), such as “let’s go.”

For those meanings, use -(으)니까:

Intended meaning	Natural Korean
It is raining, so take an umbrella.	비가 오니까 우산을 가져가세요.
We have no time, so let’s go quickly.	시간이 없으니까 빨리 가요.
It is cold, so close the window.	추우니까 창문을 닫으세요.

This is not simply a difference in English translation. Both endings can correspond to “because” or “since,” but Korean chooses between them partly according to what the speaker does in the result clause.

Examples in Context

Korean	English	Note
바빠서 못 가요.	I can’t go because I’m busy.	A personal state causes inability.
늦어서 죄송합니다.	I’m sorry for being late.	A very common apology pattern.
맛있어서 다 먹었어요.	I ate it all because it was delicious.	The result is in the past.
피곤해서 집에 있을 거예요.	I’ll stay home because I’m tired.	The final clause expresses the future.
비가 많이 와서 길이 막혔어요.	Traffic was bad because it rained heavily.	Different subjects are understood in the two clauses.
지하철이 빨라서 자주 타요.	I often take the subway because it is fast.	A quality explains a habit.
날씨가 좋아서 공원에 갔어요.	The weather was nice, so I went to the park.	English “so” naturally reflects the result.
너무 긴장해서 이름을 잊어버렸어요.	I was so nervous that I forgot the name.	“So … that” can be a natural translation.
한국 친구가 있어서 한국어를 자주 써요.	I have a Korean friend, so I use Korean often.	있다 becomes 있어서.
휴대폰이 없어서 연락을 못 했어요.	I couldn’t contact you because I didn’t have my phone.	없다 becomes 없어서.
주말이라서 식당이 붐벼요.	The restaurant is crowded because it is the weekend.	Noun + 이라서.
매운 음식을 잘 못 먹어서 이건 조금 어려워요.	I’m not good at eating spicy food, so this is a little difficult for me.	The first clause explains a personal difficulty.
설명이 쉬워서 바로 이해했어요.	The explanation was easy, so I understood right away.	A direct cause and result.
도와주셔서 감사합니다.	Thank you for helping me.	Honorific 시 appears in 주셔서.

English translations do not always need the word “because.” Depending on emphasis and context, -아/어서 may sound more natural as “so,” “as,” “since,” or a “so … that” construction.

Common Mistakes

Choosing the ending from the final syllable, not the final vowel

• Wrong: 먹아서 배가 불러요.
• Right: 먹어서 배가 불러요.
• Why: The stem 먹- has ㅓ as its last vowel, so it takes -어서. The final consonant ㄱ does not determine the choice.

Keeping the polite ending inside the first clause

• Wrong: 바빠요서 못 가요.
• Right: 바빠서 못 가요.
• Why: Remove -요 and attach the connective ending to the stem. Politeness is shown by the ending of the whole sentence.

Putting the result before the reason

• Wrong for this pattern: 못 가요 바빠서.
• Right: 바빠서 못 가요.
• Why: The basic -아/어서 structure places the cause before the result. Sentence-final -아서 does occur in conversation, but it follows different discourse patterns and is not the beginner model.

Using it before a command

• Wrong in the standard learner pattern: 비가 와서 우산을 가져가세요.
• Right: 비가 오니까 우산을 가져가세요.
• Why: The second clause is a request or command. -(으)니까 naturally supports instructions based on the speaker’s reason.

Using it before a “let’s” suggestion

• Wrong in the standard learner pattern: 배가 고파서 밥을 먹읍시다.
• Right: 배가 고프니까 밥을 먹읍시다.
• Why: 먹읍시다 proposes a shared action. Use -(으)니까 before a proposal.

Overmarking past time in the reason clause

• Avoid as a beginner: 어제 아팠어서 학교에 못 갔어요.
• Preferred core pattern: 어제 아파서 학교에 못 갔어요.
• Why: In the standard A2 pattern, the adverb 어제 and final past verb 갔어요 already establish the time. Keep the connective clause unmarked for tense.

Usage Notes

-아/어서 is neutral and extremely common in both polite conversation and ordinary writing. It often presents the connection as a straightforward cause and consequence rather than as a strongly argued justification. This makes it natural for explanations of events, inability, emotions, thanks, and apologies.

Several fixed social expressions use it frequently:

• 늦어서 죄송합니다. — I’m sorry for being late.
• 도와주셔서 감사합니다. — Thank you for helping me.
• 만나서 반가워요. — Nice to meet you. (Literally, “I’m glad because I meet/have met you.”)

English speakers sometimes try to translate “because” with one Korean ending in every situation. Instead, first ask what the second clause does. If it reports a consequence, -아/어서 is often a good fit. If it tells someone what to do or suggests a shared action, choose -(으)니까.

Beyond the Basics / Advanced Use

You do not need to master all of the following at A2, but these uses explain forms you will encounter later.

Cause and sequence can overlap

With action verbs, -아/어서 can also connect two actions in a natural sequence, often with a relationship like “do X and then do Y” or “do Y by doing X.” The same person normally performs both closely connected actions.

• 집에 가서 쉬었어요. — I went home and rested.
• 시장에 가서 과일을 샀어요. — I went to the market and bought fruit.
• 버스를 타고 가서 친구를 만났어요. — I went by bus and met my friend.

In 집에 가서 쉬었어요, “going home” is not merely a logical reason; it creates the setting for resting. Context decides whether a causal translation (“because”) or a sequential translation (“and then”) is better.

Compare -아/어서 and -(으)니까

The distinction is not that one always means “because” and the other always means “since.” Both express reasons. The following guide is more useful:

Situation	-아/어서	-(으)니까
Neutral cause leading to a result	Very common	Possible, often more explicitly reasoned
Apology or thanks	Especially natural	Usually not the default choice
Command or request in result	Avoid in the core rule	Natural
“Let’s” suggestion in result	Avoid in the core rule	Natural
Speaker stresses a basis for judgment	Possible in some contexts	Often preferred

Compare:

• 비가 와서 길이 막혔어요. — It rained, so traffic was bad. (reported consequence)
• 비가 오니까 우산을 가져가세요. — Since it is raining, take an umbrella. (reason for an instruction)

Real conversation is more flexible than beginner rules, and speakers sometimes use forms that a basic textbook discourages. Treat the restriction as a reliable production rule: it will keep your Korean natural while you build a feel for wider conversational usage.

Honorific and negative material stays inside the clause

Other grammatical material can appear before the connective ending:

• 선생님이 오셔서 모두 조용해졌어요. — The teacher came, so everyone became quiet. (오시다 → 오셔서)
• 시간이 없어서 택시를 탔어요. — I took a taxi because I had no time.
• 잘 몰라서 대답을 못 했어요. — I could not answer because I did not know well.

The connective still attaches to the final predicate of the cause clause, after an honorific marker such as -시- when one is needed.

Practice Tips

1. Transform familiar polite forms. Make pairs such as 가요 → 가서, 먹어요 → 먹어서, 공부해요 → 공부해서, and 더워요 → 더워서. This reinforces that both endings use the same vowel and contraction rules.
2. Build cause-result chains from daily life. Finish prompts such as 피곤해서…, 시간이 없어서…, and 날씨가 좋아서… with one true result. Keep the reason first and put tense and politeness on the final verb.
3. Sort by sentence type. When you make a command or “let’s” sentence, deliberately switch to -(으)니까. Contrasting 비가 와서 길이 막혔어요 with 비가 오니까 우산을 가져가세요 is more effective than memorizing either ending alone.

Related Concepts

• Prerequisite: Polite Ending -아/어요 — supplies the same vowel rules and contractions used to form -아/어서.
• Next step: Reason with -(으)니까 — expresses reasons and can introduce commands, requests, and proposals.