The hardest part of kanji for many learners is not writing them, it is that one character can have two, three, or more readings. There is a logic to it.
Two sources
When Japan adopted Chinese characters, it kept both an approximation of the Chinese sound and the existing Japanese word for the same idea.
On'yomi (the Chinese reading)
Used mostly in compound words built from two or more kanji. 学校 (gakkou, school) uses the on reading of each character.
Kun'yomi (the Japanese reading)
Used when the kanji stands alone or with hiragana endings. 学ぶ (manabu, to learn) uses the kun reading of the same character.
One character, two readings
yama, a mountain (kun reading)
Fujisan, Mount Fuji (san, the on reading)
hito, a person (kun)
sannin, three people (nin, the on reading)
The rough rule
A kanji on its own, or followed by hiragana, usually takes the kun reading. A kanji glued to another kanji in a compound usually takes the on reading. It is a tendency, not a law, and names break it constantly, which is why even Japanese readers sometimes ask how a name is pronounced.
Furigana to the rescue
Small kana printed above a kanji, called furigana, spell out the reading. You will see them in children's books, manga, and anywhere a reading is not obvious. They are the most reliable way to learn how a specific word is actually read.
Test one of these examples in the translator and see how it comes out.
Open the Translator