The overview page taught you how each irregular class changes. This page teaches the single most useful meta-rule of the whole system: when it changes. Irregular stems are not restless — they sit perfectly still before most endings and only transform in front of specific ones. So instead of memorizing dozens of conjugated forms word by word, you learn to ask one question about the ending that is coming: what does it start with? Sort the ending into one of three buckets, and the stem's behavior is decided for you.
The core idea: it's the ending that decides
Every Korean ending begins in one of three ways, and that opening determines whether an irregular stem fires:
- Consonant-initial endings — -고, -지, -게, -다, -네, -지만. These attach directly to the bare stem. Irregulars do not change. This is the safe zone.
- 으-initial endings — -(으)면, -(으)니까, -(으)ㄴ, -(으)ㄹ, -(으)세요. The linking 으 hooks onto a final consonant. Most irregulars fire here.
- 아/어 vowel-initial endings — -아/어요, -았/었어요, -아/어서, -아/어도. These are the strongest trigger; nearly every irregular changes.
Watch a single ㅂ-irregular, 덥다 ("is hot"), move through all three environments:
오늘은 덥고 습해요.
oneureun deopgo seupaeyo
Today is hot and humid. (env 1: before -고, NO change — 덥고)
너무 더우니까 창문 좀 열까요?
neomu deo-unikka changmun jom yeolkkayo
Since it's so hot, shall we open the window? (env 2: before -(으)니까, ㅂ → 우 — 더우니까)
밖이 너무 더워요.
bakki neomu deowoyo
It's so hot outside. (env 3: before -어요, ㅂ → 우 then fuses — 더워요)
One stem, three faces, and the ending picked each one. That is the entire system in miniature.
Environment 1: consonant endings — always safe
When the ending starts with a consonant, it clamps straight onto the dictionary stem and nothing shifts. This is true across every irregular class, which makes consonant endings the easiest place in Korean to conjugate — you literally do nothing.
라디오 듣고 있어요.
radio deutgo isseoyo
I'm listening to the radio. (듣다 + -고 → 듣고, ㄷ untouched)
여기서 걷지 마세요.
yeogiseo geotji maseyo
Please don't walk here. (걷다 + -지 → 걷지, ㄷ untouched)
이 문제는 어렵지만 재미있어요.
i munjeneun eoryeopjiman jaemi-isseoyo
This problem is hard but fun. (어렵다 + -지만 → 어렵지만, ㅂ untouched)
Notice 듣고 keeps its ㄷ even though the very same verb becomes 들어요 before a vowel. The ㄷ → ㄹ rule is real, but it is asleep until a vowel or 으 wakes it.
Environment 2: 으-initial endings — the linker attaches
A large family of endings has an optional 으 at the front: -(으)면 ("if"), -(으)니까 ("because"), -(으)ㄴ / -(으)ㄹ (attributives), -(으)세요 (honorific). That 으 exists precisely to bridge onto a final consonant — and reaching for the final consonant is exactly what wakes an irregular stem. Here is where ㅂ softens to 우, ㄷ turns to ㄹ, ㅅ drops, and ㄹ vanishes.
조금만 더 걸으면 도착해요.
jogeumman deo georeumyeon dochakaeyo
If we walk just a little more, we'll arrive. (걷다 → 걸으면, ㄷ → ㄹ)
이렇게 더운 날에는 나가기 싫어요.
ireoke deo-un nareneun nagagi sireoyo
On a hot day like this, I don't want to go out. (덥다 → 더운, ㅂ → 우 before -(으)ㄴ)
The ㄹ-irregular deserves special attention here, because it does the opposite of the others: instead of the 으 attaching, the stem's ㄹ drops entirely before -(으)세요. This is why "Where do you live?" is 사세요, not ×살으세요.
어디에 사세요?
eodie saseyo
Where do you live? (살다 → 사세요, ㄹ drops before -(으)세요)
Environment 3: 아/어 endings — the strongest trigger
The vowel-initial endings built on -아/어 are where irregulars fire most dramatically, because there is no linking 으 to cushion the join — the ending's bare vowel meets the stem head-on and forces a fusion. This is the environment behind the present tense -아/어요, the past -았/었어요, and the connective -아/어서.
라디오를 매일 들어요.
radioreul maeil deureoyo
I listen to the radio every day. (듣다 → 들어요)
학교까지 걸어서 가요.
hakgyokkaji georeoseo gayo
I walk to school. (걷다 → 걸어서)
이 김치가 생각보다 매워요.
i gimchiga saenggakboda maewoyo
This kimchi is spicier than I expected. (맵다 → 매워요)
Which vowel — 아 or 어? The harmony rule
Environment 3 hides a second decision: does the ending appear as 아 or as 어? The answer is Korean's vowel harmony rule, and it is mechanical:
- If the stem's last vowel is ㅏ or ㅗ (bright), use 아.
- For anything else, use 어.
- 하다 is the lone exception: 하 + 여 → 해.
저는 부산에 살아요.
jeoneun busane sarayo
I live in Busan. (살- has ㅏ → 아 → 살아요)
지금 열심히 공부해요.
jigeum yeolsimhi gongbuhaeyo
I'm studying hard right now. (하다 → 해요)
For ㅂ-irregulars there is a wrinkle worth previewing: once ㅂ becomes 우, the ending is always 어 (우 + 어 → 워), regardless of the stem's original vowel — so even a bright-vowel stem lands on 워, never ×와. That is the heart of the ㅂ irregular page.
The whole system on one card
| Ending environment | Examples | Does an irregular fire? | 덥다 → |
|---|---|---|---|
| -고, -지, -게, -다, -지만 | No — attach directly | 덥고, 덥지 |
| -(으)면, -(으)니까, -(으)ㄴ/ㄹ, -(으)세요 | Yes — most classes | 더우면, 더운 |
| -아/어요, -았/었-, -아/어서 | Yes — strongest trigger | 더워요, 더워서 |
The payoff is that you never again memorize a conjugation as a fixed lump. You look at the incoming ending, sort it into one of three buckets, and apply the class rule (or, in bucket 1, apply nothing). English has no parallel to this — its irregular verbs change unpredictably and in every environment (I walk / I walked but I think / I thought, with no phonological logic). Korean localizes the whole thing to the stem–ending seam and tells you exactly when to look.
Common Mistakes
1. Firing the change before a consonant ending. Consonant endings are the safe zone — the stem does not move.
❌ 음악을 드러고 있어요.
Wrong — before -고 there is no change. 듣다 → 듣고.
✅ 음악을 듣고 있어요.
eumageul deutgo isseoyo
I'm listening to music.
2. Applying the ㅂ softening before -고. Even ㅂ-irregulars sit still before consonant endings.
❌ 오늘은 더우고 습해요.
Wrong — env 1 is safe; ㅂ stays. → 덥고.
✅ 오늘은 덥고 습해요.
oneureun deopgo seupaeyo
Today is hot and humid.
3. Treating -(으)세요 as a consonant ending for a ㄹ stem. The hidden 으 makes ㄹ drop.
❌ 어디에 살으세요?
Wrong — -(으)세요 is an 으-environment; the ㄹ of 살다 drops. → 사세요.
✅ 어디에 사세요?
eodie saseyo
Where do you live?
4. Getting the harmony vowel wrong in environment 3. A bright-vowel stem takes 아, not 어.
❌ 저는 부산에 살어요.
Wrong — 살- has ㅏ (bright), so it takes 아. → 살아요.
✅ 저는 부산에 살아요.
jeoneun busane sarayo
I live in Busan.
Key Takeaways
- Irregulars fire based on the ending, sorted into three environments.
- Environment 1 (consonant: -고, -지, -게, -다): no change — always safe.
- Environment 2 (으-initial: -(으)면, -(으)니까, -(으)세요): most classes fire; watch that -(으)세요 counts here (사세요, not ×살으세요).
- Environment 3 (아/어 vowel-initial): the strongest trigger; also apply vowel harmony to pick 아 vs 어.
- Ask "what kind of ending is coming?" instead of memorizing each form.
Now practice Korean
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Start learning Korean→Related Topics
- Regular vs Irregular Predicates: The Big PictureTOPIK 1 — The seven irregular predicate classes are not chaos — each is a small, predictable sound change keyed to the stem's FINAL letter, and adjectives conjugate by the exact same machinery as verbs.
- Irregular Predicates at a Glance (Reference Table)TOPIK 2 — One-screen reference for all eight irregular classes — the trigger, the change, a model verb with its 아/어-form and 으-form, and a regular look-alike to guard against over-generalizing each class.
- The ㅂ Irregular: 덥다 → 더워요TOPIK 1 — How stem-final ㅂ softens to 우 and fuses with the ending — the class that covers almost every weather and sensation adjective — plus the rule that the ending vowel here is ALWAYS 어 → 워, never 와.
- Vowel Harmony: Choosing -아 vs -어TOPIK 1 — One rule fixes the shape of every -아/어 ending: if the stem's LAST vowel is ㅏ or ㅗ (bright), use 아; for anything else, use 어. The single memorized exception is 하다 → 해.
- The ㄷ Irregular: 듣다 → 들어요TOPIK 2 — How stem-final ㄷ mutates to ㄹ before a vowel- or 으-initial ending — 듣다 → 들어요, 들으면, 들으세요 — while staying put before consonant endings (듣고, 듣는).