kun and on reading

Question

Hello I recently read the difference between the kun or on readings and understood it, now I know in which cases to use one or the other reading, but I still have a question. For example: the kanji 日 have 2 on readings - にち and じつ(and also two kun reading), then which readings should I use? How do I know which and when I have to use?

Answer 1

You should learn correct reading for each word. In relatively small number of cases, multiple readings of given word are possible.

Kanji 日 has also reading に often used in 日本 ("Japan") and its compounds: 日本語 ("Japanese language"), 日本人 ("Japanese person") etc.

All on readings of kanji 日 come from Middle Chinese *nyit, so /t/ in Early Middle Japanese forms of these readings (nit, zit) is regularly expected. ni developed as irregular shortening of nit.

_{Middle Japanese and Early Modern Japanese is known to have had syllable-final /t/ in Sino-Japanese vocabulary, so despite hiragana/katakana spelling of these readings there was no vowel after /t/.}