How do you know where to read the kanji as On Yomi and where Kun Yomi? (Excuse me if these are not the correct terms)
Generally speaking, it depends on whether the word is of Chinese origin (pronounced with Onyomi) or native Japanese (pronounced with Kunyomi). But there are lots of exceptions to this rule, so ultimately you just have to memorize them all.
For beginners, however, as a rule of thumb, if a verb or adjective has a hiragana suffix (called okurigana), it’s pronounced in kunyomi. For example:
綺麗 (pretty, clean, onyomi, “kiREI”)麗らか (bright, beautiful, kunyomi, “URAraka”)
食べる (to eat, kunyomi, “TAberu”)飲む (to drink, kunyomi, “NOmu”)飲食する* (to eat and drink, onyomi, “INSHOKU suru”)
*する is basically “to do” and Japanese’s way of shoehorning words into becoming a verb. inshoku on its own is “eating and drinking” as a NOUN and suru effectively makes it “to do eating and drinking.” It’s not a suffix, unlike the other two verbs.
Notice how even though the same kanji are used and the meanings are very similar, the pronunciations are different depending on the word’s origin.
For nouns (at beginners level), usually if there’s only a single kanji, it’s pronounced in kunyomi.
東 (east, kunyomi, “HIGASHI”)南 (south, kunyomi, “MINAMI”) 西 (west, kunyomi, “NISHI”) 北 (north, kunyomi, “KITA”)
But wait, if you put them together to form a compound word, they become onyomi!
東南 (southeast, onyomi, “TOUNAN”)東北 (northeast, onyomi, “TOUHOKU”)
Like everything else, there are always EXCEPTIONS. Some kanjis only have onyomi pronunciations so they’ll be pronounced as such even if they are standalone:
肉 (meat, onyomi, “NIKU”)駅 (train station, onyomi, “EKI”)
Are there any kunyomi compound words, then, you ask? Absolutely!
青空 (blue sky, kunyomi, “AOZORA”)毛玉 (furball, kunyomi, “kedama”)
And things gets a little harder from here. You know what else there are? Compound words that can be pronounced in BOTH kunyomi and onyomi:
人気 (popularity, onyomi, “NINKI”)人気 (human presence/sign of life, kunyomi, “HITOKE”)
草原 (grassland, onyomi or kunyomi, “SOUGEN” or “KUSAHARA”)While the two pronunciations have the same meaning, there’s a slight difference in nuance. Sougen is like a prairie wheres Kusahara is more modest in size.)
Confused yet? Because there’s more! There are compound words where the first half is in kunyomi and the second half in onyomi, and vice versa:
毎年 (every year, onyomi+kunyomi, “MAITOSHI”)長年 (many years, kunyomi+onyomi, “NAGANEN”)
And then, some words are #JustJapaneseThings and there isn’t a kanji with a corresponding meaning for it, so the Japanese essentially ‘spell’ them out using completely unrelated kanjis for their phonetic sounds only. These type of words are called ateji:
矢張り (as expected/nonetheless, kunyomi, “YAHARI”)Literal meaning of kanji: “stretch an arrow”
兎に角 (anyway/at any rate, onyomi, “TONIKAKU”)Literal meaning of kanji: “horns for rabbit”
In the case of words where there ARE corresponding kanji, but the native Japanese pronunciations simply won’t fit into the normal kunyomi/onyomi system, they become words with their own unique pronunciations called jukujikun:
昨日 (yesterday, irregular, “KINOU”)二十歳 (20 years old, irregular, “HATACHI”)お土産 (souvenir, irregular, “oMIYAGE”)
What if it’s something the Japanese don’t have a word for? Before katakana was all the rage, both ateji and jukujikun were used:
仏蘭西 (France, onyomi, “FURANSU”)Literal meaning of kanji: “Buddha-orchid-west”)
煙草 (Tabacco, irregular, “TABAKO”)
Both are now more commonly known as フランス and タバコ respectively.
And if a word/phrase is a 4-kanji idiom (yojijukugo), it’s likely pronounced in onyomi:
唯一無二 (one and only, onyomi, “YUITSUMUNI”)花鳥風月 (literally “flower-bird-wind-moon” meaning “beautiful nature” which is later used to describe people who appreciate nature by drawing or writing poems about it, onyomi, “KACHOUFUUGETSU”)
There are many more, and as you progress you’ll find lots more irregular words that you can only learn from memorization.
