Palatalization (구개음화)
ㄷ and ㅌ change to ㅈ and ㅊ when followed by the vowel 이 (i-sound).
Palatalisation (구개음화) is a more specific sound rule: when the final consonants ㄷ or ㅌ are followed by the vowel 이 (the 'i' sound), they shift to ㅈ or ㅊ respectively. This happens mostly across morpheme boundaries — typically when a suffix beginning with 이 attaches to a stem ending in ㄷ or ㅌ.
Example: 같이 (together) is spelled 같 + 이 but pronounced 'ga-chi' because the final ㅌ of 같 palatalises into ㅊ before the 이. 굳이 (deliberately) is pronounced 'gu-ji' for the same reason. Palatalisation is less frequent than liaison or nasalisation, but recognising it is essential because the most common Korean adverb 같이 follows this rule. Learners who pronounce 같이 as 'gat-i' rather than 'ga-chi' will be understood but sound noticeably foreign.
ㄷ + 이 → ㅈ (Palatalization)
When ㄷ is the 받침 and the next syllable is 이 (or starts with the i-vowel), ㄷ changes to ㅈ. 굳이[구지]: 굳(ㄷ) + 이 → [구지]. 맏이[마지]: 맏(ㄷ) + 이 → [마지].
ㅌ + 이 → ㅊ (Palatalization)
When ㅌ is the 받침 and the next syllable is 이, ㅌ changes to ㅊ. 같이[가치]: 같(ㅌ) + 이 → [가치]. 붙이다[부치다]: 붙(ㅌ) + 이 → [부치다].
Cultural & Language Note
구개음화 only happens across a morpheme boundary, which makes it a quiet grammar lesson disguised as a pronunciation rule. 같이 becomes 'gachi' because 이 is a separate particle attached to 같. But in a single indivisible word like 잔디 (lawn), the ㄷ stays put. Korean pronunciation rules frequently encode where one meaningful unit ends and the next begins.
Common Mistakes to Avoid
Over-applying is the usual error: learners palatalise every ㄷ or ㅌ before 이, including inside single morphemes, and say 'janji' for 잔디. It also does not apply when 이 is part of the same root. When in doubt, ask whether the 이 is a particle or ending attached to something — if it is, palatalise; if it is just part of the word, leave it alone.