はじめまして Cassie です。よろしくおねがいします。 私が間違えたら直してください。日本語 は 難しいでも和楽しです。 みなさん ありがと！

Please explain this to me Hello! I am just now starting to learn Japanese, so forgive me for being so ignorant. I am wondering why certain kanji can be written one way but pronounced many ways such as 日 (ひ) pronounced "hi", then 日本 (にほん) which contains 日 but this time it's pronounced "ni" Is this common in Japanese? For kanji to be pronounced differently depending on what it's next to? And is there any way to know the rule for that if so? Or is it all memorizing? Thank you very much! :)