口 and 川口
I didn't know it was called rendaku 連濁 in Japanese. And I didn't know the rule of it.
However, I can say rendaku words properly and naturally as a Japanese native.
Ah, I found a term for this happening in Japanese. It's called rendaku. 連濁 - "sequential voicing"
I was thinking it was something along those lines. Isn't there a linguistic term for this, too?
Yes, this happens a lot. To put it in slightly technical terms, the first consonant becomes voiced (meaning that the vocal cords vibrate). Consonants come in voiceless/voiced pairs, for example t/d p/b s/z k/g f/v and so on. So kuchi become guchi and tachi becomes dachi when the kanji is being pronounced after another kanji. But this doesn't happen all the time, so you kind of just have to memorize it.
I don´t know if that -always- happens, but I see that everywhere. "達" for example... The kun reading is -たち, but in the word 友達 the た is replaced with a だ ---> ともだち