Like the first answer explained, it depends on the KUN reading of the Kanji and the ON reading. The Kun reading is the original japanese word. It's composed of just the kanji alone.
Ex. 人= hito, which means person ( The kanji alone means you have to read the kun reading which is the original word for it)
The ON reading on the other hand is used mostly for compound Kanjis.
EX. 日本人＝NIHONJIN which means Japanese person ( There are more than one kanji which means you must read it as the ON reading, chinese reading)
Although the KUN reading serves for other things as you may learn later on. When you see that there is a kanji first and then its followed by the Hiragana alphabet, you read the kanji as the KUN reading. This is used for Adjectives, verbs, and in special cases for nouns. I'll give the example with another kanji that's a verb.
Ex. 終わる＝ OWARU which means to finish. ( Notice that the first character is a kanji and the rest is followed by hiragana. When there is such an instance the kanji is read with the KUN reading always which in this case was 'O'. The same applies to adjectives and SOME nouns.)
There are some cases where the compound kanjis together are read in their KUN reading but very rarely. The most common instance is when reading the name of a Japanese person. Others just have to be memmorized along with the vocabulary writing and reviewing process.
Ex1. 田中さん＝TANAKA SAN which means Mr. Tanaka ( Notice that this is a japanese name being read with KUN instead of ON reading. The easiest way to know to read it as KUN is when at the end it has -san, -chan, -sama etc.. which are types of respect forms to different people.
Ex.2 青空= AOSORA which means Blue Sky. ( This type of kanji although a compound is read with the kun reading. There are rarely any occasions like this but it just has to be memmorized. The ON yomi would otherwise be SEI KARA which means absolutley nothing. These are used for rare nouns)
I hope this helped and kanji seems hard but with dedication it can be overcomed. Good luck!/がんばってください！
· 1 decade ago