Based on your rules, you would not be able to predict 人気 ninki, 人類 jinrui Why is popularity "plain" and humanity is "identity"? You just have to memorize ALL

Based on your rules, you would not be able to predict 人気 ninki, 人類 jinrui Why is popularity "plain" and humanity is "identity"? You just have to memorize ALL of the compounds, there's no other way