르-irregular: 르 → ㄹㄹ before vowel-starting endings
Verbs whose stem ends in 르 — like 모르다 (not know), 빠르다 (be fast), 부르다 (call/sing), 다르다 (be different), 고르다 (choose), 자르다 (cut) — undergo a doubling change before vowel-starting endings. The 으 in 르 drops, the previous syllable picks up an extra ㄹ as its bottom consonant, and the new ㄹ becomes the start of the next syllable. So 모르다 + -아요 → 몰라요 (lit. 'mol-la-yo'). The pattern is consistent and very common — most verbs ending in 르 are 르-irregular, with only a tiny handful of regular exceptions.
The doubling mechanic — step by step
Take 모르다 (to not know). Drop -다 to get the stem 모르-. The polite present requires an -아/어 ending. The 르-irregular says: (1) drop the 으 vowel from 르, (2) add an extra ㄹ to the previous syllable as a bottom consonant (받침), so 모 becomes 몰, (3) the leftover ㄹ + 아/어 attaches as 라/러 depending on the previous vowel. Result: 모르 + 아요 → 몰 + 라요 → 몰라요 = 'I don't know.' Compare other verbs: 빠르다 (be fast) → 빨라요. 부르다 (call/sing) → 불러요. 자르다 (cut) → 잘라요. 고르다 (choose) → 골라요. 다르다 (be different) → 달라요.
Vowel choice: 라 vs. 러
The doubled syllable picks up the appropriate -아 vs. -어 vowel. If the syllable BEFORE 르 has ㅏ or ㅗ as its main vowel, use 라. Otherwise use 러. So 모르 (the previous syllable 모 has ㅗ) → 몰라. 빠르 (빠 has ㅏ) → 빨라. 자르 (자 has ㅏ) → 잘라. 고르 (고 has ㅗ) → 골라. 다르 (다 has ㅏ) → 달라. Going the other way, 부르 (부 has ㅜ, not ㅏ/ㅗ) → 불러. 흐르다 (to flow, 흐 has ㅡ) → 흘러요. 누르다 (to press, 누 has ㅜ) → 눌러요. 기르다 (to raise/grow, 기 has ㅣ) → 길러요.
Past tense: 라/러 + -았/었어요
Past tense follows the same mechanic with -았/었어요. 모르다 → 몰랐어요 (didn't know). 빠르다 → 빨랐어요 (was fast). 부르다 → 불렀어요 (called/sang). 다르다 → 달랐어요 (was different). 자르다 → 잘랐어요 (cut). The -았 vs. -었 choice mirrors the -라 vs. -러 choice — same vowel-harmony rule. You'll hear and use these constantly: 저는 몰랐어요! = 'I didn't know!' 그 가수는 노래를 잘 불렀어요. = 'That singer sang the song well.'
Before consonant endings: 르 stays whole
The doubling kicks in ONLY before vowel-starting endings (-아/어요, -았/었어요, etc.). Consonant-starting endings keep 르 intact. 모르다 + -지만 → 모르지만 (not 몰르지만). 모르다 + -고 → 모르고. 빠르다 + -지만 → 빠르지만. 부르다 + -고 → 부르고. For -(으)면 (vowel-starting after 으): the 으 attaches normally with no doubling — 모르면 (NOT 몰르면). For -(으)ㄹ 수 있다: same — 모를 수 있어요 (NOT 몰를). The trigger is specifically -아/어 family endings.
A few regular 르-stems
Most 르 verbs are irregular, but a small set of regular 르-stems exist. The most-used: 들르다 (to drop by, stop in) → 들러요 — looks irregular but you don't double the ㄹ; the rule is simpler: 으 drops before vowel endings, like other 으-stems. Compare: 들르다 + -어요 → 들러요 (regular 으-drop, no doubling). 부르다 + -어요 → 불러요 (irregular doubling). The two end up looking similar in writing but the underlying mechanic differs. At A2 just be aware that 들르다 is the regular oddball; almost everything else (모르, 빠르, 부르, 다르, 고르, 자르, 흐르, 누르, 기르, 오르 — to climb/rise) follows the doubling pattern.
