Non-universal counters and reading 7 and 9


Q1. Why are these counters like 人(にん), 日(にち) and 歳(さい) not universal?

  1. 人(にん) is impossible to be joined with 1(いち) or 2(に)?
  2. 日(にち) is impossible to be joined with 2, 3, 4, 5, 6, 7, 8, 9, 10, 14, 20 or 24?
  3. 歳(さい) is impossible to be joined with 20(にじゅう)?

Q2. When do we read 7 and 9 as "しち" and "く" rather than "なな" and "きゅう"?

