경음화 (硬音化) is a phenomenon in which for certain Korean words, ㅈ in the middle is pronounced ㅉ, ㅂ is pronounced ㅃ, ㄱ is pronounced ㄲ, and so on. You have to remember these kind of words one by one in order not to pronounce them wrong.
문법 or ‘grammar’ is pronounced [문뻡]. →ㅂ pronounced as ㅃ
문자 or ‘text message’ is pronounced [문짜]. → ㅈ pronounced as ㅉ
거스름돈 (change/small money) is pronounced [거스름똔]. → ㄷ pronounced as ㄸ
성격 (personality, character) is pronounced [성:껵]. →ㄱ pronounced asㄲ
장난감 (toy) is pronounced [장난깜]. →ㄱ pronounced asㄲ
It reminds me of a song from 2012 by Teen Top: 나랑 사귈래 (Be My Girl). because at 3:10, the lyrics goes like this: 넌 누구꺼? 넌 내꺼. (You belong to whom? You belong to me.).
The grammatically correct way to write 누구꺼 is 누구 거.
Even though it’s written 누구 거, it’s actually pronounced [누구꺼]. And also, the grammatically correct way to write 내꺼 is 내 거, but it’s pronounced [내꺼].
누구 거 (whose is it?) is pronounced [누구꺼]. →ㄱ pronounced asㄲ
내 거 (mine) is pronounced [내꺼]. →ㄱ pronounced asㄲ
According to this tweet by 국립국어원, the polite and correct way to say ‘this is mine.’ is ‘이건 제 거예요’.
But remember that 제 거 is actually pronounced [제꺼]. ###