In short

Some stems change their final consonant when a vowel-initial ending arrives: 덥다 -> 더워요 (deowoyo, it’s hot), 듣다 -> 들어요 (deureoyo, I listen), 짓다 -> 지어요 (jieoyo, I build). And a ㄹ-final stem does the opposite - it drops its ㄹ in front of ㄴ, ㅂ and ㅅ: 살다 -> 삽니다, 사세요, 사는. Nothing here is optional, and a look-alike like 받다 or 입다 stays perfectly regular.

The rule

Step 1 - The change happens BEFORE the ending, not to the ending

You already know the 아/어 engine (lesson 10). An irregular class does not change that engine; it changes the stem first, and then the normal engine runs on the changed stem. So the question is always the same two-part question:

1. What class is this stem in? (You learn that with the word, like a gender in German.)
2. What does the ending it meets start with?

Step 2 - Three ending classes, and that is all

Every ending you will ever attach falls into one of three buckets, and the bucket decides whether anything happens:

• Vowel-initial: -아/어요, -았/었어요, -아/어서. These fire ㅂ, ㄷ and ㅅ.
• -(으)- endings: -(으)면 (if), -(으)세요 (please do), -(으)니까 (because), -(으)ㄴ, -(으)ㄹ (modifier endings). These also fire ㅂ, ㄷ and ㅅ, because the 으 is a vowel.
• Plain consonant-initial: -고 (and), -지만 (but), -게, -겠-, -습니다. Nothing happens to a ㅂ, ㄷ or ㅅ stem - it is written exactly as in the dictionary: 덥고 (deopgo), 듣고 (deutgo), 짓고 (jitgo). (A ㄹ stem is the one that still reacts here, to ㄴ, ㅂ and ㅅ - see Step 6.)

Step 3 - ㅂ 불규칙: the ㅂ becomes 우

Drop the ㅂ and put 우 in its place; the 우 then merges with the following vowel. Because 우 is not ㅏ or ㅗ, the ending is always 어, so the 해요 form always comes out as -워요:

덥 + 어요 -> 더우 + 어요 -> 더워요

Two members break the pattern in one place only: 돕다 (dopda, to help) and 곱다 (gopda, to be lovely) give 도와요 (dowayo) and 고와요 (gowayo), not *도워요. Everything else about them is normal (도우면, 도운).

Most ㅂ-final descriptive verbs are in this class; most ㅂ-final action verbs are not.

Step 4 - ㄷ 불규칙: the ㄷ becomes ㄹ

In front of a vowel the final ㄷ is rewritten as ㄹ, and then the ending attaches normally:

듣 + 어요 -> 들 + 어요 -> 들어요

This class is small - 듣다 (deutda, to listen), 걷다 (geotda, to walk), 묻다 (mutda, to ask) - and it is surrounded by regular look-alikes (받다, 닫다, 믿다) that you simply have to keep apart.

Step 5 - ㅅ 불규칙: the ㅅ disappears and nothing contracts

The ㅅ is deleted, but the two vowels left behind do not merge. This is the part learners get wrong:

짓 + 어요 -> 지 + 어요 -> 지어요 (never *져요) 낫 + 아요 -> 나 + 아요 -> 나아요 (never *나요)

Step 6 - ㄹ 탈락: the ㄹ runs away from ㄴ, ㅂ, ㅅ

A ㄹ-final stem is regular in front of a vowel (살아요, 살았어요), but it deletes the ㄹ in front of an ending beginning with ㄴ, ㅂ or ㅅ - and none of the endings taught so far begins with one, because all four triggers arrive later: -ㅂ니다 (lesson 27), -(으)세요 (lesson 21), the -는/-(으)ㄴ modifier (lesson 24) and -(으)니까 (lesson 28). Read the ㄹ column as forms to recognise now and to produce from those lessons on; what you say today is 살아요 and 살았어요. On top of that, a ㄹ stem never takes the 으 of an -(으)- ending - it behaves like a vowel stem:

살 + -ㅂ니다 -> 삽니다 - 살 + -세요 -> 사세요 - 살 + -는 -> 사는 - 살 + -니까 -> 사니까

But 으-less does not mean ㄹ-less. In -(으)면 the ending really starts with ㅁ, which is not ㄴ/ㅂ/ㅅ, so the ㄹ survives and only the 으 is dropped: 살면 (salmyeon), 만들면 (mandeulmyeon). This is not an irregularity you memorise word by word - every ㄹ stem does it.

Forms & patterns

The trigger table - which ending class fires which change:

ㅂ 불규칙 - and the regular ㅂ stems underneath:

ㄷ 불규칙 - and its three regular look-alikes:

ㅅ 불규칙 - and its three regular look-alikes:

ㄹ 탈락 - there are no exceptions and no regular look-alikes:

Two details worth pinning down. First, a ㄹ stem takes -ㅂ니다, not -습니다, exactly like a vowel stem: 삽니다, never *살습니다. Second, the whole 살다 family in one line: 살아요 - 삽니다 - 사세요 - 사는 - 사니까 - 삽시다 (sapsida) - 살면 - 살고.

Examples

Common mistakes

• 날씨가 덥어요. (Nalssiga deobeoyo.) -> 날씨가 더워요. (Nalssiga deowoyo.) - 덥다 is ㅂ-irregular, so the ㅂ becomes 우 before a vowel. Only regular ㅂ stems keep it: 입다 -> 입어요.
• 음악을 듣어요. (Eumageul deudeoyo.) -> 음악을 들어요. (Eumageul deureoyo.) - ㄷ-irregular: the ㄷ is rewritten as ㄹ in front of a vowel. The look-alike 받다 does not change - 받아요, never *발아요.
• 집을 져요. (Jibeul jyeoyo.) -> 집을 지어요. (Jibeul jieoyo.) - After a ㅅ-irregular stem the vowels never contract. Same for 낫다: 나아요, not *나요.
• 서울에 살습니다. (Seoure salseumnida.) -> 서울에 삽니다. (Seoure samnida.) - A ㄹ stem drops its ㄹ before ㅂ and then takes -ㅂ니다 like a vowel stem.
• 김밥을 만들으세요. (Gimbabeul mandeureuseyo.) -> 김밥을 만드세요. (Gimbabeul mandeuseyo.) - A ㄹ stem never takes the 으 of -(으)세요, and the ㄹ itself falls out before the ㅅ.

Tips

• The ㄹ mnemonic: ㄴ, ㅂ, ㅅ. The ㄹ is shy and runs away from those three consonants - and it never lets an 으 stand next to it. Everything else (vowels, -고, -지만, -면) leaves it alone.
• Guess by word class, then verify. Almost every ㅂ-final adjective is irregular (덥다, 춥다, 맵다, 쉽다, 어렵다, 무겁다, 가깝다), while ㅂ-final action verbs are usually regular (입다, 잡다, 씹다). 돕다 and 굽다 (grill, roast) are the well-known action-verb exceptions - 도와요 (not *도워요) and 구워요.
• Watch the twin 묻다. 묻다 meaning to ask is irregular (물어요), but 묻다 meaning to bury is regular (묻어요). Same spelling, different class - the sentence tells you which.
• Learn the 해요 form, not the rule. A good dictionary prints the 아/어 form beside the entry. If you store 덥다 as the pair “덥다 - 더워요” from day one, you never have to run the rule at speaking speed.