A outranks B on some quality, and the gap between them is wide.
序 · The move
1Build the comparison first: A 比 B, then the bare quality Vs.Is there a real comparison? 得多 measures a gap, so a yardstick (比 B, or context) has to be present.
2Hang 得多 straight off the quality: Vs + 得多.Nothing between Vs and 得, and no 很 before Vs — 很 cancels the comparison.
3Read it back: the gap is large.If the gap is small, 一點 belongs here, not 得多.
例 · Examples
1他哥哥his older brother的年紀's age比他compared to him大big / old (of age)得多by a lot。
His older brother is much older than he is.
界 · Boundary
比…一點
得多 says the gap is large; 一點 says the gap is small. Same comparison, opposite size of distance.
Plain 得 rates how one thing turned out (跑得快). Vs得多 rates the distance between two things — 得多 only makes sense once a comparison is on the table.
比…多了
得多 sits tight on the quality (大得多). 多了 trails the whole comparison (大多了) and carries a touch of surprise; 得多 just states the size.
他比他哥哥很大得多 → 他比他哥哥大得多 (no 很 inside a comparison; 得多 already carries the degree)
他哥哥比他得多大 → 他哥哥比他大得多 (得多 follows the quality, never precedes it)
這本書貴得多 (with no comparison anywhere) → 這本書比那本貴得多 (得多 measures a gap, so a B must be present)
English folds the size into the adjective up front ('much taller', 'far older'). Chinese keeps the bare quality and tacks the size on behind as 得多, after the standard set by 比.