In Japanese, there are two ways to pronounce Kanji: “Onyomi (Chinese way of pronunciation )” and “Kunyomi (Japanese way of pronunciation)”. For example, 海 (sea) has /umi/ and /kai/.
Kanji is imported to Japan around 65 A.D with bunch of goods from China. Until Heian period (794-1185), Kanji was used only among higher educated men. Japan did not have a writing system until Kanji was imported.
The appearance of Kunyomi and Onyomi in Japanese started merging meaning of Kanji with words that already existed in Japanese during Heian period. Hiragana (another Japanese character in modern days) is sometimes called “cursive from” that is a transformed character of Kanji created by women.
How do we know which pronunciation is Onyomi or Kunyomi? Here is some tips for kanji pronunciation. This is NOT based on linguistic aspects.
1. The words include the sound of /n/ and palatalised sound are usually Onyomi.
e.g. /sha/, /sho/, and /cho/ in /shacho/ (Company president), /Shoko/ (evidence proof).
2. Three syllable words are usually Onyomi.
e.g. /kimochi/ (feeling), /kaeru/ (flog).
3. The words start with a sonorant and /r/ are usually Onyomi.
e.g. /bouken/ (advebture), /dainshi/ (male), /zibun/ (self).
4. 2 syllable Onyomi words only exist on /tsu/, /chi/,/ku/,/ki/,/u/,/i/, and /n/ ending. However, Kunyomi 2 syllable words also exist on these sounds ending.
e.g. /yuki/ (snow) is Kunyomi, but /mitsu/ (honey) is Onyomi.
5. The polite form suffix /o/ is attached to Japanese words (Kunyomi), and /go/ to Chinese words(Onyomi).
e.g. /go han/ (meal, rice) is Onyomi. /o hashi/ (chopsticks) is kunyomi.
These observation would be useful to memorize the pronunciation of Japanese words. However, these observations do not have linguistics evidence, I will research this issue linguistically.