When To Use On’yomi & Kun’yomi

Textfugu.com

1. When a kanji is on its own, often times you’ll use the くん reading (though there are exceptions to this).
2. When a kanji is combined with another kanji, and no hiragana is attached, you’ll probably be using the おん reading. This is jukugo.
3. A lot of the time, you’ll have to have prior knowledge of a particular word before you can know the pronunciation of it. Often times you canguess the pronunciation of a word / kanji based on your studies / a few indicators, but in the end preparedness and experience is key."