Rules for irregular conjugation ‘ㄹ’ for word stems that end with ㄹthe way Korean textbooks written in Japanese explain it. Related post: Basic Korean Conjugation (conjugation 1, 2, and 3).
1. For conjugation 1 & 2, if the sentence ending/suffix starts with ㄴ,ㅂ, or ㅅ, or ‘badchim ㄹ’, the ㄹ of the word stem falls out.
만들다 and -ㅂ니다 (starts with ㅂ, conjugation 1)
만들 + -ㅂ니다 = 만드 + -ㅂ니다 = 만듭니다
열다 and -세요 (starts with ㅅ, conjugation 2)
열 + -(으)세요 = 여 + -세요 = 여세요
알다 + -(으)ㄹ까요? (starts with ‘badchim ㄹ’, conjugation 2)
알 + -ㄹ까요? = 아 + ㄹ까요? = 알까요?
2. For conjugation 1, if the suffix/sentence ending doesn’t start with ㄴ,ㅂ, ㅅ, or badchim ㄹ, the ㄹ stays.
들다 and -지만 (conjugation 1, doesn’t start with ㄴ,ㅂ, ㅅ, or badchim ㄹ)
들 + -지만 = 들지만
3. For conjugation 2, if the sentence ending/suffix doesn’t start with ㄴ,ㅂ, ㅅ, or badchim ㄹ, the ㄹ of the word stem doesn’t’ fall out, and 으 is not added (remember that for regular conjugation 2, 으 is added for word stems that end with consonant).
놀다 + -(으)러 = 놀 + 러 = 놀러
흔들다 + -(으)면 = 흔들 + 면 = 흔들면
4. Conjugation 3 is the same with the regular one. Thank God.
길다 + -(아/어)도 (conjugation 3) = 길어도 ###