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].