You have a whole count and the real amount runs a little past it, the extra not worth naming.
序 · The move
1Set the round number down — a count that ends clean (一, 十, 三百).Is there a leftover beyond this whole that you don't want to spell out?
2Slot 多 in to carry the remainder; for a divisible unit it sits between number and measure (一個多), for a whole-unit count it trails the measure (十個多).Does 多 land where the leftover actually falls — inside the unit, or past the last whole one?
3Finish the measure word and noun.Is 多 reading as a remainder on top of the number, not as a free-standing 'many'?
例 · Examples
1他們聊天chat聊了chatted (action done)一個one (whole) — the round count多and a bit more on top小時hour (the measure being counted)。
數+量 names an exact count (一個小時 = one hour, flat). Wedging 多 in (一個多小時) puts a remainder on top of that count — over an hour. Same number, but 多 says the real amount runs past it.
幾 stands IN PLACE of an unknown small digit (幾個 = a few, how many). 多 adds ON TOP of a number already stated (三十多 = thirty-odd). 幾 replaces the count; 多 extends it.
✗ 多一個小時 (reads as 'one more hour') → ✓ 一個多小時 (over an hour)
✗ 一多個小時 → ✓ 一個多小時 (多 wedges between number and measure, not between number and 個 when the unit divides)
✗ 三十幾年 for 'thirty-odd years' confused with a known total → ✓ 三十多年 (a remainder past a stated thirty)
English glues the approximation to the front as a loose word ('over an hour', 'an hour or so'); Chinese fixes 多 inside the count at the exact place the leftover falls, so learners reach for a front adverb instead of wedging 多 in.