Actually, modifications to the pronunciation of 'ㄷ' and 'ㅌ' occur quite often! Adara is right that it is being used for the ease of pronunciation. But there is a simple rule when does this sound change take place. It changes in front of '이' or diphtong '여' : 1) 'ㄷ' is pronounced as 'ㅈ' 2) 'ㅌ' is pronounced as 'ㅌ'. For example : 맏이 ---> [마지] 같이 ---> [가치] 끝이 ---> [끄치] 붙이다 ---> [부치다] 붙여요 ---> [부쳐요] A more complicated case, which you can face with, involves 닫혀요 (being closed), which is being pronounced as 다쳐요 (the same as 'get hurt') You would probably ask - "Why? The second syllable in the word 닫혀요 is '혀', not '여"! Two things happen there. First, the combination of 'ㄷ' with 'ㅎ' in 닫혀요 results in 'ㅌ' (you know that simple rule, right? 'ㄱ' + 'ㅎ' = 'ㅋ' ; 'ㅂ' + 'ㅎ' = 'ㅍ' and so on) Second, this 'ㅌ' pronouced as if it was 'ㅊ' because of the following '여'. So, basically - 닫혀요 >>> 닽여요 >>> 다쳐요 Hope it helps!