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"?
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2020-10-05T00:38:51.243
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2020-10-05T00:47:58.793
2020-10-05T00:47:58.793
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readings numbers
Non-universal counters and reading 7 and 9