Verb stems and the 아/어/여 conjugation engine
A1 어간과 아·어·여 활용 10 / 68
In short
Every Korean verb you meet in a dictionary ends in -다. Cut off the -다 and you have the 어간 (eogan, stem) - and almost everything you will ever attach to a verb hangs off that stem. This lesson teaches the single most productive attachment of all: the 아/어/여 form, chosen by one small rule and then contracted.
The rule
Step 1 - Dictionary form = stem + 다
The form in the dictionary is 어간 + -다. The -다 is not part of the word’s meaning; it is just the citation ending.
- 가다 (gada, to go) -> stem 가
- 먹다 (meokda, to eat) -> stem 먹
- 마시다 (masida, to drink) -> stem 마시
- 읽다 (ikda, to read) -> stem 읽
You never say 가다 in a normal sentence. You say 가요, 갔어요, 가서, 가고 - stem plus something. So the first habit to build is: see -다, mentally delete it.
Step 2 - Two shapes of stem
A stem either ends in a vowel (가, 마시, 배우, 하) or in a 받침 (먹, 읽, 앉, 좋). That difference decides whether the ending contracts, and it also decides the (으) in endings like -(으)면 and -(으)세요 later on.
Step 3 - Choose 아 or 어 by the stem’s LAST vowel
This is the whole selection rule, and it has exactly three lines:
- The stem’s last vowel is ㅏ or ㅗ -> attach -아.
- Anything else (ㅓ, ㅜ, ㅡ, ㅣ, ㅐ, ㅔ, ㅕ, ㅚ, …) -> attach -어.
- 하다 is irregular in its own way: 하 + 여 -> 해.
Only the last vowel of the stem counts, never the first. 가르치다 (gareuchida, to teach) has ㅏ in the first syllable, but the last vowel of the stem 가르치 is ㅣ, so it is 가르쳐 (gareucheo), not *가르차.
Stem + -아 / -어 / -여 -> the 아/어 form
Step 4 - Contract, obligatorily
After a 받침 nothing happens - the ending just sits there: 먹 + 어 = 먹어, 읽 + 어 = 읽어, 앉 + 아 = 앉아, 좋 + 아 = 좋아.
After a vowel the two vowels merge, and the merged spelling is the only correct one in normal writing. *가아요 and *마시어요 are simply errors. The full table is in Forms & patterns below - learn it as a table, not as a theory.
Step 5 - 으 is not an irregularity, it is part of this rule
A stem that ends in ㅡ - an open 으 syllable, with no 받침 - drops that ㅡ before 아/어, and then the harmony is decided by the vowel of the syllable before it. A ㅡ carrying a 받침 never drops, so 늦다 and 힘들다 simply take -어 by Step 3 (늦어요 neujeoyo, 힘들어요 himdeureoyo):
Stem in 으 + 아/어 -> ㅡ deleted, harmony read from the previous syllable
- 아프다 (apeuda, to hurt): 아프 -> 아ㅍ, previous vowel ㅏ -> 아파 (apa)
- 예쁘다 (yeppeuda, to be pretty): 예쁘 -> 예ㅃ, previous vowel ㅖ -> 예뻐 (yeppeo)
- 쓰다 (sseuda, to write): nothing precedes, so the default 어 wins -> 써 (sseo)
This is regular. Every 으-stem behaves this way, so do not file it with the real irregular classes (lesson 17 and lesson 39), which is where ㅂ, ㄷ, ㅅ, ㄹ, 르 and ㅎ stems are handled.
Step 6 - Why this one form is worth memorising
The 아/어 form is not just for the polite ending. It is the input to a whole family of patterns you will meet over the next two levels - build it once, reuse it everywhere.
Forms & patterns
The selection rule, with worked stems:
| Last vowel of the stem | Ending | Dictionary form | Stem | 아/어 form |
|---|---|---|---|---|
| ㅏ | -아 | 받다 (batda, to receive) | 받 | 받아 (bada) |
| ㅗ | -아 | 좋다 (jota, to be good) | 좋 | 좋아 (joa) |
| ㅓ | -어 | 먹다 (meokda, to eat) | 먹 | 먹어 (meogeo) |
| ㅣ | -어 | 씻다 (ssitda, to wash) | 씻 | 씻어 (ssiseo) |
| ㅜ | -어 | 웃다 (utda, to laugh) | 웃 | 웃어 (useo) |
| 하 | -여 | 하다 (hada, to do) | 하 | 해 (hae) |
The contraction table - the heart of this lesson:
| Stem | + ending | Result | Romanization | What merged |
|---|---|---|---|---|
| 가 (가다, to go) | + 아 | 가 | ga | ㅏ + ㅏ -> ㅏ (absorbed) |
| 오 (오다, to come) | + 아 | 와 | wa | ㅗ + ㅏ -> ㅘ |
| 보 (보다, to see) | + 아 | 봐 | bwa | ㅗ + ㅏ -> ㅘ |
| 배우 (배우다, to learn) | + 어 | 배워 | baewo | ㅜ + ㅓ -> ㅝ |
| 주 (주다, to give) | + 어 | 줘 | jwo | ㅜ + ㅓ -> ㅝ |
| 마시 (마시다, to drink) | + 어 | 마셔 | masyeo | ㅣ + ㅓ -> ㅕ |
| 되 (되다, to become) | + 어 | 돼 | dwae | ㅚ + ㅓ -> ㅙ |
| 서 (서다, to stand) | + 어 | 서 | seo | ㅓ + ㅓ -> ㅓ (absorbed) |
| 지내 (지내다, to get along) | + 어 | 지내 | jinae | ㅐ + ㅓ -> ㅐ (absorbed) |
| 켜 (켜다, to switch on) | + 어 | 켜 | kyeo | ㅕ + ㅓ -> ㅕ (absorbed) |
| 하 (하다, to do) | + 여 | 해 | hae | ㅏ + ㅕ -> ㅐ |
으 탈락 - the ㅡ drops, then look one syllable back:
| Dictionary form | Stem | Vowel before the 으 | 아/어 form | Romanization |
|---|---|---|---|---|
| 아프다 (to hurt) | 아프 | ㅏ -> 아 | 아파 | apa |
| 바쁘다 (to be busy) | 바쁘 | ㅏ -> 아 | 바빠 | bappa |
| 예쁘다 (to be pretty) | 예쁘 | ㅖ -> 어 | 예뻐 | yeppeo |
| 기쁘다 (to be glad) | 기쁘 | ㅣ -> 어 | 기뻐 | gippeo |
| 쓰다 (to write, to use) | 쓰 | (none) -> 어 | 써 | sseo |
| 크다 (to be big) | 크 | (none) -> 어 | 커 | keo |
Where the 아/어 form is consumed downstream:
| Pattern | Meaning | Built on 가 / 먹어 / 해 | Lesson |
|---|---|---|---|
| -아/어요 | polite informal ending | 가요, 먹어요, 해요 | L12 |
| -았/었- | past tense | 갔어요, 먹었어요, 했어요 | L16 |
| -아/어서 | because; and then | 가서, 먹어서, 해서 | L28 |
| -아/어 주다 | do it for someone | 가 주다, 먹어 주다, 해 주다 | L33 |
| -아/어 보다 | try doing it | 가 보다, 먹어 보다, 해 보다 | L35 |
| -아/어도 | even if | 가도, 먹어도, 해도 | L49 |
| -아/어지다 | become, get -er | 커지다, 예뻐지다, 좋아지다 | L37 |
Note the 띄어쓰기: the ENDINGS 요, -서, -도 attach with no space, but the AUXILIARY VERBS 주다 and 보다 are separate words - 해 주세요, 먹어 보세요.
Examples
- 저는 학교에 가요. (Jeoneun hakgyoe gayo.) - I go to school.
- 친구가 집에 와요. (Chinguga jibe wayo.) - My friend is coming home.
- 저는 물을 마셔요. (Jeoneun mureul masyeoyo.) - I drink water.
- 동생이 한국어를 배워요. (Dongsaengi hangugeoreul baewoyo.) - My younger sibling learns Korean.
- 매일 아침을 먹어요. (Maeil achimeul meogeoyo.) - I eat breakfast every day.
- 저녁에 텔레비전을 봐요. (Jeonyeoge tellebijeoneul bwayo.) - In the evening I watch television.
- 오늘은 배가 아파요. (Oneureun baega apayo.) - Today my stomach hurts.
- 요즘 아주 바빠요. (Yojeum aju bappayo.) - I am very busy these days.
- 여기에서 편지를 써요. (Yeogieseo pyeonjireul sseoyo.) - I write letters here.
- 저는 매일 운동해요. (Jeoneun maeil undonghaeyo.) - I exercise every day.
Common mistakes
- 학교에 가아요. (Hakgyoe gaayo.) -> 학교에 가요. (Hakgyoe gayo.) - The contraction is obligatory, not optional. ㅏ + ㅏ is always written as one ㅏ.
- 저는 밥을 먹아요. (Jeoneun babeul meogayo.) -> 저는 밥을 먹어요. (Jeoneun babeul meogeoyo.) - The stem 먹 has ㅓ, not ㅏ or ㅗ, so the ending is 어. Only ㅏ and ㅗ take 아.
- 선생님이 한국어를 가르차요. (Seonsaengnimi hangugeoreul gareuchayo.) -> 선생님이 한국어를 가르쳐요. (Seonsaengnimi hangugeoreul gareucheoyo.) - Harmony reads the LAST vowel of the stem 가르치 (ㅣ), never the first.
- 머리가 아프어요. (Meoriga apeueoyo.) -> 머리가 아파요. (Meoriga apayo.) - An 으-stem deletes its ㅡ, and the harmony then comes from 아 in 아프.
- 저는 의사가 되요. (Jeoneun uisaga doeyo.) -> 저는 의사가 돼요. (Jeoneun uisaga dwaeyo.) - 되 + 어 contracts to 돼. Even Korean natives slip here; the rule is that 되 alone can never carry 요.
Tips
- One question, one answer. Look at the last vowel of the stem and ask only: “Is it ㅏ or ㅗ?” Yes -> 아. No -> 어. 하다 -> 해. There is no fourth case.
- The 돼 / 되 trick. Wherever you are unsure between 되 and 돼, substitute 하 and 해. If 해 sounds right, write 돼; if 하 sounds right, write 되. So 돼요 (= 해요), but 되고 (= 하고).
- 하다 pays the biggest dividend. Thousands of Korean verbs are a noun plus 하다 (공부하다, 운동하다, 사랑하다), and every one of them ends in 해 - one memorised form unlocks the largest verb class in the language. Written as one word: 공부해요, never *공부 해요.
- Do not conjugate for tense here. Unlike English, the 아/어 form itself carries no tense or person. 먹어 is just “eat” as raw material; tense, politeness and mood all come from what you stack on top of it.