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].
How It Works
구개음화 means 'palatalisation', and it is named after the hard palate — the roof of your mouth just behind the teeth ridge. ㄷ and ㅌ are made by touching the tongue to the ridge. The vowel 이 is made with the tongue raised towards the palate, slightly further back. When ㄷ or ㅌ is immediately followed by 이, the tongue is already heading for the palate, so the consonant is pulled along with it and lands as ㅈ or ㅊ instead.
The correspondence preserves the plain/aspirated distinction: plain ㄷ becomes plain ㅈ, and aspirated ㅌ becomes aspirated ㅊ. So 굳이 → [구지] and 맏이 → [마지] for ㄷ, while 같이 → [가치] and 붙이다 → [부치다] for ㅌ.
One restriction matters and is often missed. This only happens across a morpheme boundary — that is, when the 이 belongs to a separate grammatical piece such as a suffix or particle, not when it is part of the same root. In 잔디 ('lawn') the ㄷ and 이 sit inside a single morpheme, so no palatalisation occurs and it stays [잔디]. This is why the rule feels inconsistent to learners who have not been told about the boundary condition: the phonetic environment alone is not sufficient.
Where You'll Hear It
같이 is by far the most important word here. It means 'together' or 'with', and it appears in invitations constantly: 같이 가요 ('let's go together'), 같이 먹어요 ('let's eat together'). It is pronounced [가치], and a learner saying [가티] will be understood but will sound noticeably off in one of the friendliest, most-used phrases in the language.
붙이다 ('to attach') shows the rule inside a verb, and it is the kind of word that turns up in practical instructions — attaching a sticker, a stamp, a label. 해돋이 ('sunrise') is a seasonal favourite: watching the first 해돋이 of the new year is a widespread custom, so the word circulates heavily every January.
굳이 is worth knowing because it is common in casual speech and hard to translate. It carries a sense of 'going out of one's way to' or 'if you really insist', often with a shade of mild reluctance — 굳이 그렇게 해야 돼요? ('do you really have to do it that way?'). Learners meet it in conversation long before it appears in textbooks, and [구지] is the only pronunciation they will hear.
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.
How to Practice This Lesson
Lead with 같이. It is the one word from this lesson you will use daily, so drill [가치] until it is automatic, inside full phrases rather than alone: 같이 가요, 같이 먹어요, 같이 공부해요. Building it into set phrases means you never have to apply the rule consciously mid-sentence.
Keep the two correspondences straight by pairing them with their source consonants rather than memorising four separate words: ㄷ+이 gives ㅈ, ㅌ+이 gives ㅊ. Plain stays plain, aspirated stays aspirated.
Finally, note the boundary condition rather than ignoring it. Write 같이 [가치] and 잔디 [잔디] next to each other. Same letters in sequence, different outcome, because one crosses a morpheme boundary and the other does not. Seeing the counterexample early prevents you from over-applying the rule later.