Nasalization (비음화)
Stop consonants ㄱ, ㄷ, ㅂ become nasal ㅇ, ㄴ, ㅁ before nasal consonants ㄴ or ㅁ.
Nasalisation (비음화) changes the stop consonants ㄱ, ㄷ, ㅂ into the nasals ㅇ, ㄴ, ㅁ when they meet a following nasal consonant ㄴ or ㅁ. The most common context where you will encounter this is the formal verb ending -ㅂ니다/습니다, which is always pronounced -ㅁ니다/슴니다 even though it is always spelled with ㅂ.
Example: 학년 (school year) is spelled hak + nyeon but pronounced hang + nyeon. The ㄱ at the end of 학 changes to ㅇ before the ㄴ of 년. Similarly 합니다 sounds like 'ham-ni-da', not 'hap-ni-da'. This rule is one of the most consistent in Korean — it applies essentially every time you have a stop final consonant followed by a nasal initial. Pay attention to formal-style sentences in particular, since they trigger nasalisation in almost every verb.
비음화 — Nasalization Rule
When ㄱ, ㄷ, or ㅂ is followed by a nasal consonant ㄴ or ㅁ, the stop consonant changes to the corresponding nasal: ㄱ→ㅇ, ㄷ→ㄴ, ㅂ→ㅁ.
비음화 — The Three Pairs
Remember these three pairs: ㄱ(g/k) → ㅇ(ng), ㄷ(d/t) → ㄴ(n), ㅂ(b/p) → ㅁ(m). All three become the nasal that shares the same place of articulation in the mouth.