알다 ("to know") and 모르다 ("to not know") are antonyms you learn on day one — and, conveniently, they are also two of the cleanest examples of Korean's two most common irregular classes. Put them side by side and you get a free lesson in contrast: 알다 is a ㄹ-irregular (its stem-final ㄹ vanishes before certain endings — 압니다, 아니까), while 모르다 is a 르-irregular (its 르 doubles the ㄹ before 아/어 — 몰라요). One antonym pair, two entirely different irregular systems. And note the deeper symmetry: just as 있다's negative is the separate word 없다, 알다's negative is the separate word 모르다 — Korean draws its most basic negatives from dedicated lexical items, not from an 안 prefix.
Two stems, two irregular systems
- 알다 — stem 알-, ends in the batchim ㄹ. The ㄹ drops before endings beginning with ㄴ, ㅂ, ㅅ (and archaic 오): 아니까, 아세요, 압니다, 아는. Before 아/어 and 으-endings the ㄹ stays: 알아요, 알면. And ㄹ-stems never take the 으 buffer.
- 모르다 — stem 모르-, a 르-irregular. Before an 아/어 ending only, the 으 of 르 drops and an extra ㄹ pushes back onto the previous syllable: 모르 + 아 → 몰라. Everywhere else the stem is perfectly regular: 모르면, 모르니까, 모릅니다.
The side-by-side paradigm
| Form / ending | 알다 (know) | 모르다 (not know) |
|---|---|---|
| 해요체 present | 알아요 arayo | 몰라요 mollayo |
| 합니다체 present | 압니다 (ㄹ drops) amnida | 모릅니다 moreumnida |
| Past | 알았어요 arasseoyo | 몰랐어요 mollasseoyo |
| Future — (으)ㄹ 거예요 | 알 거예요 al geoyeyo | 모를 거예요 moreul geoyeyo |
| Connective — so / because (-아서) | 알아서 araseo | 몰라서 mollaseo |
| Connective — if / when (-면) | 알면 almyeon | 모르면 moreumyeon |
| Connective — since (-(으)니까) | 아니까 (ㄹ drops) anikka | 모르니까 moreunikka |
| Honorific (-(으)세요) | 아세요 (ㄹ drops) aseyo | 모르세요 moreuseyo |
| Attributive present · past · prospective | 아는 · 안 · 알 aneun · an · al | 모르는 · 모른 · 모를 moreuneun · moreun · moreul |
Read the two irregulars off the table. On the 알다 side, every ㄹ-drop cell is triggered by a following ㄴ/ㅂ/ㅅ (압니다, 아니까, 아세요, 아는), and the ㄹ survives before vowels (알아요, 알아서). On the 모르다 side, the doubled ㄹ appears in exactly the two 아/어 cells (몰라요, 몰랐어요, 몰라서) and nowhere else — 모르면, 모르니까, 모릅니다 stay regular.
죄송한데 저도 이 동네를 잘 몰라요.
joesonghande jeodo i dongnereul jal mollayo
Sorry, I don't really know this neighbourhood either. (몰라요, the 르 doubling)
그 정도는 저도 알아요.
geu jeongdoneun jeodo arayo
Even I know that much. (알아요, ㄹ kept before a vowel)
부모님도 이 사실을 아세요?
bumonimdo i sasireul aseyo
Do your parents know about this too? (아세요 — ㄹ drops before ㅅ)
저는 원래 길을 잘 몰라서 지도를 항상 켜 놔요.
jeoneun wollae gireul jal mollaseo jidoreul hangsang kyeo nwayo
I'm bad with directions, so I always keep the map open. (몰라서, cause)
The idioms that matter most: 알겠어요 / 모르겠어요
Here is the piece textbooks under-teach. In real conversation Koreans very often prefer the -겠- forms over the bare present:
| Form | Reading | What it really means |
|---|---|---|
| 알겠어요 / 알겠습니다 | algesseoyo / algetseumnida | "I understand / got it / I'll do that" — acknowledgement |
| 모르겠어요 | moreugesseoyo | "I'm not sure / I don't know" — the polite default |
| 알게 됐어요 | alge dwaesseoyo | "I came to know / found out" (no one's plan) |
The -겠- here is not the future tense. It signals tentativeness and, in 알겠습니다, willing compliance — "understood, I'll see to it." Bare 알아요 states a flat fact ("I know it"); 알겠어요 marks the moment of understanding ("ah, now I get it") and softens the claim. On the negative side, bare 몰라요 can sound blunt or dismissive, so the softened 모르겠어요 is the everyday way to say "I don't know" politely. This is a genuine register fact, not decoration: answering a stranger's question with a flat 몰라요 lands more curtly than the hedged 모르겠어요.
아, 이제 알겠어요. 설명 감사합니다.
a, ije algesseoyo. seolmyeong gamsahamnida
Ah, I get it now. Thanks for the explanation. (알겠어요 = the click of understanding)
글쎄요, 저도 잘 모르겠어요.
geulsseyo, jeodo jal moreugesseoyo
Hmm, I'm not really sure either. (softened, polite 'I don't know')
네, 알겠습니다. 바로 처리하겠습니다.
ne, algetseumnida. baro cheorihagetseumnida
Yes, understood. I'll take care of it right away. (알겠습니다 = compliance)
한참 뒤에야 그 사람 사정을 알게 됐어요.
hancham dwie-ya geu saram sajeong-eul alge dwaesseoyo
It was only much later that I found out that person's situation.
Common Mistakes
1. Inserting 으 or keeping ㄹ before 니까/ㅂ니다. ㄹ-stems take no 으, and the ㄹ drops before ㄴ/ㅂ.
❌ 제가 잘 알으니까 걱정 마세요.
Wrong — no 으, and ㄹ drops before ㄴ: it's 아니까.
✅ 제가 잘 아니까 걱정 마세요.
jega jal anikka geokjeong maseyo
I know it well, so don't worry.
2. Forming the formal as ×알습니다 / ×알ㅂ니다. The ㄹ drops before ㅂ → 압니다.
❌ 저는 그 답을 알습니다.
Wrong — ㄹ drops before ㅂ니다: 압니다.
✅ 저는 그 답을 압니다.
jeoneun geu dabeul amnida
I know the answer. (formal)
3. Doubling the ㄹ of 모르다 outside 아/어. The 르-rule fires only before 아/어; 니까 leaves the stem regular.
❌ 길을 몰라니까 택시를 탔어요.
Wrong — 니까 doesn't trigger the doubling: 모르니까.
✅ 길을 모르니까 택시를 탔어요.
gireul moreunikka taeksireul tasseoyo
I didn't know the way, so I took a taxi.
4. Answering a superior's instruction with 알아요. Compliance is 알겠습니다 / 알겠어요.
❌ (상사에게) 네, 알아요.
Marked — to a superior's instruction, the acknowledgement is 알겠습니다, not 알아요 ('I already know').
✅ 네, 알겠습니다.
ne, algetseumnida
Yes, understood. (I'll do it)
Key Takeaways
- 알다 = ㄹ-irregular: the ㄹ drops before ㄴ/ㅂ/ㅅ (아니까, 압니다, 아세요, 아는) and stays before vowels (알아요, 알아서); no 으 buffer ever.
- 모르다 = 르-irregular: the ㄹ doubles only before 아/어 (몰라요, 몰랐어요, 몰라서); everywhere else it is regular (모르면, 모르니까, 모릅니다).
- The pair is lexically antonymic — 모르다 is the standalone negative of 알다, like 없다 for 있다.
- In conversation, prefer 모르겠어요 (softened "I don't know") and use 알겠어요 / 알겠습니다 for "I understand / will comply"; the -겠- is tentativeness and volition, not future tense.
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Start learning Korean→Related Topics
- ㄹ-Irregular Predicates (ㄹ 탈락): Full TableTOPIK 2 — The complete reference table for ㄹ-stem verbs and adjectives, whose stem-final ㄹ drops before endings beginning with ㄴ, ㅂ, ㅅ, or the honorific 시 (mnemonic ㄴ·ㅂ·ㅅ·시) and which never take the 으 buffer — 살다 → 삽니다, 사세요, 사니까, 사는, 산.
- 르-Irregular Predicates (르 불규칙): Full TableTOPIK 2 — The complete lookup grid for the 르-irregular class — before an 아/어 ending the 으 of 르 drops and an extra ㄹ pushes back onto the previous syllable (모르다 → 몰라요, 부르다 → 불러요), with 라/러 set by harmony — plus the 으-drop imposters (따르다·치르다) and the separate 러-irregular (이르다·푸르다).
- Irregular vs Regular: The Look-Alike Master TableTOPIK 3 — The cheat-card for the question learners actually ask — 'this verb ends in ㄷ/ㅅ/ㅂ/ㅎ/르, does it inflect irregularly?' A single minimal-pair table sets each irregular next to a regular verb with the same final consonant, so you can see that irregularity is lexical, not spelling-based, and that the safe default for an unknown verb is REGULAR.
- Conjugation Sheet: 있다 / 없다 (exist / have / be located)TOPIK 1 — A side-by-side quick sheet for the antonym pair 있다 and 없다 — Korean's one verb for 'there is / is at / have' and its dedicated negative — with the verbal -는 attributive (있는/없는), the honorific split 계시다 vs 있으시다, and the two aspectual auxiliaries -고 있다 and -아/어 있다.