If you learn only one Korean irregular early, make it the 르 irregular (르 불규칙). It governs a cluster of the most common words in the language — 모르다 ("not know"), 다르다 ("be different"), 빠르다 ("be fast"), 부르다 ("call / sing / be full") — and it produces the striking doubled-ㄹ shape you have surely already heard: 모르다 → 몰라요. Once you see why the ㄹ doubles, the whole class becomes predictable.
The trigger is narrow, which is good news: only the 아/어 endings set it off. Everything else — consonant endings, 으-endings — leaves the stem completely regular. So the 르 irregular is really a rule about one specific junction.
The rule
When an 아/어 ending attaches to a stem ending in 르, two things happen at once:
- the 으 of 르 drops, and
- an extra ㄹ appears as the batchim of the preceding syllable —
giving a doubled ㄹ (ㄹ + 러/라).
| Stem |
| Result | Doubled ㄹ |
|---|---|---|---|
| 모르- | 모르 + 아요 | 몰라요 | 모 gains ㄹ batchim → 몰 + 라 |
| 부르- | 부르 + 어요 | 불러요 | 부 gains ㄹ batchim → 불 + 러 |
저는 그 사람 이름을 몰라요.
jeoneun geu saram ireumeul mollayo
I don't know that person's name.
택시 좀 불러 주세요.
taeksi jom bulleo juseyo
Please call a taxi for me.
The reframe: the second ㄹ is pushed back, not invented
The doubled ㄹ looks alarming until you see that neither ㄹ comes from nowhere. Start from 부르 + 어. The 르 keeps its ㄹ, but the 으 vowel drops, so the ㄹ has no vowel to sit on. Rather than vanish, that ㄹ splits its job: one ㄹ pushes back to become the batchim of the previous syllable (부 → 불), and a second ㄹ stays to launch the ending (러). The result is 불 + 러 → 불러요.
- 부르 + 어요 → 불 + 러요 → 불러요 (never ×부러요)
- 모르 + 아요 → 몰 + 라요 → 몰라요 (never ×모라요)
라 vs 러 is ordinary vowel harmony
Whether you get 라 or 러 is decided by the 아/어 harmony of the syllable before 르 — the one that gains the ㄹ batchim. Bright vowel (ㅏ or ㅗ) → 라; anything else → 러.
| Verb | Meaning | Syllable before 르 | 해요 form |
|---|---|---|---|
| 모르다 | not know | 모 (ㅗ, bright) | 몰라요 |
| 빠르다 | be fast | 빠 (ㅏ, bright) | 빨라요 |
| 다르다 | be different | 다 (ㅏ, bright) | 달라요 |
| 고르다 | choose | 고 (ㅗ, bright) | 골라요 |
| 자르다 | cut | 자 (ㅏ, bright) | 잘라요 |
| 부르다 | call / sing / be full | 부 (ㅜ, dark) | 불러요 |
| 흐르다 | flow | 흐 (ㅡ, dark) | 흘러요 |
이 기차가 생각보다 훨씬 빨라요.
i gichaga saenggakboda hwolssin ppallayo
This train is much faster than I expected.
형제인데 성격이 완전히 달라요.
hyeongjeinde seonggyeogi wanjeonhi dallayo
They're brothers, but their personalities are completely different.
선물을 뭐로 살지 아직 못 골랐어요.
seonmureul mworo salji ajik mot gollasseoyo
I still haven't picked what to buy as a gift.
미용실에서 머리를 짧게 잘랐어요.
miyongsireseo meorireul jjalge jallasseoyo
I got my hair cut short at the salon.
이 강물은 서해로 흘러요.
i gangmureun seohaero heulleoyo
This river flows out to the West Sea.
부르다 is worth its own line
부르다 packs three everyday meanings — "call (out / summon)," "sing," and "be full (after eating)" — and all three run through the same 불러 form. It is one of the most-used verbs you'll conjugate, so drill it until 불러요 is automatic.
노래방에서 노래를 신나게 불렀어요.
noraebang-eseo noraereul sinnage bulleosseoyo
We sang our hearts out at the noraebang.
많이 먹어서 배가 너무 불러요.
mani meogeoseo baega neomu bulleoyo
I ate so much that I'm really full.
Only 아/어 triggers it — everything else is regular
This is the half of the rule that keeps you from over-doubling. Before a consonant ending or an 으 ending, a 르 stem is perfectly regular: no dropped 으, no doubled ㄹ.
| Ending | 모르다 → | Doubled ㄹ? |
|---|---|---|
| -아/어요 | 몰라요 | yes |
| -고, -지 | 모르고, 모르지 | no |
| -(으)면, -(으)니까 | 모르면, 모르니까 | no |
| -는 (attributive) | 모르는 | no |
(The 으-endings show no 으 at all here, because the stem 모르 already ends in a vowel — 면 and 니까 attach straight on.)
답을 모르면 손을 드세요.
dabeul moreumyeon soneul deuseyo
If you don't know the answer, raise your hand.
제가 잘 모르니까 다시 설명해 주세요.
jega jal moreunikka dasi seolmyeonghae juseyo
I don't really know, so please explain it again.
이건 제가 아는 노래랑 완전히 다른 곡이에요.
igeon jega aneun noraerang wanjeonhi dareun gogieyo
This is a totally different song from the one I know.
Beware the imposters (르 that isn't 르-irregular)
A few common stems look like they end in 르 but actually belong to the plain 으-drop class — they just drop the 으 and take a single ㄹ, with no doubling. The most important:
- 따르다 (follow / pour) → 따라요 (not ×딸라요)
- 치르다 (pay / undergo) → 치러요 (not ×칠러요)
- 들르다 (drop by) → 들러요 (not ×들르러요… → the plain 들러요)
Telling the doublers from the non-doublers is exactly the job of the next page, 르 irregular vs 으-drop. For now, just know that 따르다 → 따라요 is not a 르-irregular.
Common Mistakes
1. Writing a single ㄹ instead of doubling. The signature error.
❌ 저는 답을 모라요.
Incorrect — the ㄹ doubles: 몰라요.
✅ 저는 답을 몰라요.
jeoneun dabeul mollayo
I don't know the answer.
2. Losing the pushed-back batchim on 부르다.
❌ 큰 소리로 이름을 부러요.
Incorrect — 부르 + 어 doubles to 불러요, not 부러요.
✅ 큰 소리로 이름을 불러요.
keun soriro ireumeul bulleoyo
I call out the name loudly.
3. Doubling before a non-아/어 ending. The doubling is triggered only by 아/어.
❌ 길을 몰르면 지도를 보세요.
Incorrect — before -(으)면 the stem is regular: 모르면.
✅ 길을 모르면 지도를 보세요.
gireul moreumyeon jidoreul boseyo
If you don't know the way, look at a map.
4. Wrong 라/러 harmony. 부르다 is dark (ㅜ), so it takes 러, not 라.
❌ 배가 불라요.
Incorrect — dark vowel ㅜ takes 러: 불러요.
✅ 배가 불러요.
baega bulleoyo
I'm full.
5. Doubling an imposter. 따르다 is a plain 으-drop verb, not a 르-irregular.
❌ 저는 형을 잘 딸라요.
Incorrect — 따르다 is 으-drop: 따라요 (single ㄹ).
✅ 저는 형을 잘 따라요.
jeoneun hyeong-eul jal ttarayo
I follow my older brother closely.
Key Takeaways
- Before an 아/어 ending, a 르 stem drops the 으 and doubles the ㄹ: 모르다 → 몰라요, 부르다 → 불러요.
- One ㄹ pushes back as the previous syllable's batchim (불, 몰); the other launches the ending (러, 라) — so a single ㄹ (×부러요) is always wrong.
- 라 vs 러 follows vowel harmony on the syllable before 르: bright (ㅏ/ㅗ) → 라, otherwise → 러.
- Only 아/어 endings trigger it — before consonant or 으 endings the stem is regular (모르고, 모르면, 모르는).
- Watch the imposters: 따르다 → 따라요 and 치르다 → 치러요 are 으-drop verbs, not doublers — see the next page.
Now practice Korean
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Start learning Korean→Related Topics
- 르-Irregular vs 으-Drop Imposters (따르다 → 따라요)TOPIK 2 — Three stems that END in 르 — 따르다, 치르다, 들르다 — are NOT 르-irregular but plain 으-drop verbs, so they take a single ㄹ (따라요), never the doubled ㄹ of a true 르-irregular (달라요). One letter apart, opposite outputs.
- The Rare 러 Irregular: 이르다 → 이르러요, 푸르다 → 푸르러요TOPIK 3 — A tiny class where the 르 stem stays intact and the -아/어 ending surfaces as 러: 이르다 'reach' → 이르러요, 푸르다 → 푸르러요, 누르다 'be deep yellow' → 누르러요. The mirror image of the 르-irregular, and a homograph trap where 이르다 and 누르다 belong to two classes at once.
- 으-Drop Harmony: 바쁘다 → 바빠요 vs 예쁘다 → 예뻐요TOPIK 2 — When a multi-syllable ㅡ stem drops its ㅡ, the choice of 아 vs 어 is decided by the vowel of the syllable IMMEDIATELY before the ㅡ. ㅏ or ㅗ there → 아 (바쁘다 → 바빠요); anything else → 어 (예쁘다 → 예뻐요).
- The 으 Drop: 쓰다 → 써요, 크다 → 커요TOPIK 1 — Any stem whose last vowel is ㅡ loses that ㅡ before an -아/어 ending. For a one-syllable ㅡ stem there is no preceding vowel, so it always defaults to 어: 쓰다 → 써요, 크다 → 커요, 끄다 → 꺼요. The most predictable of all the 'irregular' classes.
- 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.