ㄹ-irregular: drop ㄹ before ㅅ, ㄴ, ㅂ, and -오
Verbs whose stem ends in ㄹ — like 살다 (live), 알다 (know), 만들다 (make), 놀다 (play), 팔다 (sell) — drop that ㄹ before certain endings. The trigger letters are easy to memorize as 'ㅅ, ㄴ, ㅂ, -오'. So 살다 (live) becomes 살아요 (regular present) but 사세요 (honorific — ㄹ dropped before -세-), 삽니다 (formal — ㄹ dropped before -ㅂ-), 사는 (modifier — ㄹ dropped before -는). This is the FIRST irregular paradigm to learn because almost every Korean ㄹ-stem verb does it the same way, and the rules are 100% predictable.
The ㅅ, ㄴ, ㅂ, -오 trigger
When a ㄹ-stem verb meets an ending that STARTS with ㅅ, ㄴ, ㅂ, or the syllable -오 (older endings), the stem-final ㄹ simply drops. Memorize the trigger letters as the mnemonic 'ㅅ ㄴ ㅂ 오' (sometimes written as 'snobbing' in romanization aids — 'snb-o'). Examples with 살다 (live): + -세요 (honorific) → 사세요 (please live / lives — honorific). + -ㅂ니다 (formal) → 삽니다 (lives — formal). + -는 (modifier present) → 사는 (a living…). Compare with endings that DON'T start with the trigger letters — there, the ㄹ stays: 살아요 (live, polite present), 살았어요 (lived), 살고 (live, and…), 살면 (if/when one lives — and yes, ㄹ-stems also drop the 으 before -면, as you saw in lesson 20).
Most-used ㄹ-stem verbs
Some you'll encounter constantly. 살다 (to live) — 살아요 / 사세요 / 삽니다 / 사는. 알다 (to know) — 알아요 / 아세요 / 압니다 / 아는. 만들다 (to make) — 만들어요 / 만드세요 / 만듭니다 / 만드는. 놀다 (to play) — 놀아요 / 노세요 / 놉니다 / 노는. 팔다 (to sell) — 팔아요 / 파세요 / 팝니다 / 파는. 열다 (to open) — 열어요 / 여세요 / 엽니다 / 여는. 울다 (to cry) — 울어요 / 우세요 / 웁니다 / 우는. Notice the pattern is always the same: the irregular forms come ONLY before ㅅ/ㄴ/ㅂ/-오 endings.
The honorific connection
The most common 'ㅅ-trigger' you'll meet is the honorific marker -(으)시-. With a ㄹ-stem, -시- attaches directly (no 으) AND drops the ㄹ: 알다 → 아시- → 아세요? = 'Do you know?' (politely asking an elder). 살다 → 사시- → 어디에 사세요? = 'Where do you live?' (polite). 만들다 → 만드시- → 무엇을 만드세요? = 'What are you making?' (polite). When you hear someone older asking 'where do you live?', they're using 사세요, not 살아세요 or 살세요. This is the #1 ㄹ-irregular form learners need to recognize and produce.
The formal -ㅂ니다 connection
Formal-polite Korean (the news-anchor / job-interview style) uses -ㅂ니다 / -습니다. Vowel-stems take -ㅂ니다 (갑니다, 옵니다), consonant-stems take -습니다 (먹습니다, 읽습니다). What about ㄹ-stems? They DROP the ㄹ and behave like vowel-stems: 삽니다 (lives), 만듭니다 (makes), 압니다 (knows), 놉니다 (plays), 팝니다 (sells). At a Korean restaurant you'll see signs like 김치 팝니다 = 'kimchi sold here'. The form is everywhere in formal speech.
The modifier connection
When you turn a verb into a noun-modifier ('a living person', 'the thing being made'), the present-tense modifier ending -는 also triggers ㄹ-drop: 사는 사람 = 'a person who lives (somewhere)'. 만드는 음식 = 'food (that) is being made'. 아는 사람 = 'a person (I) know' = an acquaintance. 파는 가게 = 'a store that sells (it)'. You'll meet noun-modifier endings in detail later, but the ㄹ-drop pattern works the same way: ㄹ disappears before -는 because -는 starts with ㄴ.
