There’s something in Japanese that makes me a bit nervous when learning: sometimes it feels like kanji readings just change out of nowhere.
For example:
戸口 (とぐち) → “doorway” (I thought it would be とくち, but it turns into とぐち)
手紙 (てがみ) → “letter” (why not てかみ?)
一分 (いっぷん) → “one minute” (why not いちふん?)
It feels kind of random, and I never know when to expect it. Is there some rule behind these changes? Or is it just one of those “you have to memorize it” things?
These are all considered not different readings of the kanji, but sound changes produced by putting them together in a word. The first two are a phenomenon called “rendaku”, where sometimes the second kanji’s first syllable gets voiced. There’s a good tofugu article on this:
The last one with the small tsu is called “gemination” and happens when certain syllables come together that would be awkward to pronounce rapidly one after the other. You’ll get used to when it happens with practice and some more vocab.