1Name the baseline — the one share everything is counted against — and set it after 的 as B.Is B the smaller, original amount? The multiple counts up from B, not down.
2Put the number of folds before 倍.兩倍 is twice the whole, not twice more — three 倍 is three times, not four.
3Open with A 是 and, if a quality is stated, close on the bare quality after 倍.Does the line still read if you drop the quality? A是B的N倍 stands alone.
例 · Examples
1姐姐older sister的行李luggage是is我的mine (the baseline)兩倍two times重heavy。
My older sister's luggage is twice as heavy as mine.
界 · Boundary
A比B X
比 ranks A above B without saying by how much; 倍 fixes the size of the gap as a count of copies. Order vs measured multiple.
A比B重兩倍 states the excess (heavier BY two extra folds); A是B的兩倍重 states the total (two folds in all). Surplus vs whole.
姐姐的行李是我兩倍重 → 姐姐的行李是我的兩倍重 (keep 的 to pin the baseline B)
姐姐的行李兩倍我的重 → 姐姐的行李是我的兩倍重 (open with 是; the multiple is not a verb on its own here)
姐姐的行李是我的重兩倍 → 姐姐的行李是我的兩倍重 (the number+倍 sits before the quality, not after)
English says 'twice as heavy AS mine', standard last; Chinese pins the baseline first with B的 and lets the number+倍 carry the count just before the quality.