You basically have to memorise them, like with any other language (e.g. the rules and exceptions of English spelling and pronunciation). Usually it's the most c

You basically have to memorise them, like with any other language (e.g. the rules and exceptions of English spelling and pronunciation). Usually it's the most common words that have weird exceptions, because we use them enough to remember them. All the words you mention are beginner words. (There's also 人気=ひとけ, just for fun). As far as jin and nin goes... (source: 漢語からみえる世界と世間) jin is more about adjectives/nouns - describing yourself, and part of your identity. 日本人、アメリカ人、原始人、現代人 are all じん. 読書人（どくしょじん） is used to describe people who really love reading. Similarly 暇人（ひまじん）. nin is more of a plain descriptor. 管理人、運搬人、苦労人、被告人 are all にん, as well as 三人.