Kanji came from Chinese characters, so kanji usually have two pronunciations: 1) the original chinese pronunciation, called on'yomi 2) the japanese pronunciation, called kun'yomi Both are used in japanese words, usually on'yomi is used in words with two or more kanji. But some kanji have more than three or four pronunciations, for example: this kanji 言 has pronunciations: i, yu, koto, gen, gon

Yes, you are right! There are a looot of words in Japanese that are written in the same way or using the same Kanji but is pronounced differently. For example-> Learn=>学ぶ (Manabu)、School=>学校 (Gakkou). Both of the words have the same Kanji "学" but are pronounced differently. But, you can see that there is always a connection between these kind of words that use the same Kanji. For example, "to learn" and "school" are interrelated; you "learn" when you go to "school". Similarly, from your example, the work for "Heart" is "心" (kokoro) but it changes to "Shin" in "心配" (shinpai; actually it is pronounced "Shimpai"). Another example that I like is the Kanji "愛". Depending on contexts and words, this same word is pronounced as "Ai", "Kana", "Mana" etc!!! I hope my answer was helpful! If there was something from my answer that you couldn't understand, please feel free to ask me again...(^ ^)

it's the problem of reading kanji :(