You want to count a thing and the number is about to land straight on the noun.
序 · The move
1Set the number down first.Is a count what you mean?
2Reach for the noun and feel that the number won't hold it bare; fit a measure word in between.Does the measure word match this noun's kind — 本 for a bound thing, 個 when nothing better fits?
3Set the noun last, behind the measure word.Is the order number, measure, noun, with nothing missing in the middle?
例 · Examples
1一one本measure for bound things書book;一個the general measure word人person。
量詞 is the inventory the slot draws from; 數+量 is the three-slot frame that puts a chosen measure between number and noun. The measure is not free: 本 for bound things, 個 the catch-all when nothing fits closer.
✗ 一書 → ✓ 一本書
✗ 一人 → ✓ 一個人
✗ 三個書 → ✓ 三本書
English counts a noun bare (one book), so the measure word gets dropped; Chinese needs the handle between the number and the noun.