grammar → 比…還…
TSUMUGU · TBCL 3 (est.) · 語法
比…還… grammar point · tier 1 · scalar 還 in comparison (even more X than B)
· hái
Inside a 比 comparison, 還 pushes the scale one step past a baseline already high: A is even more X than B.

字源 FORM what the parts do

A road (辶) with 睘 huán lent only for the sound; the return-word worn light into the adverb hái. It sits before the quality and carries the scale one step further.

故事 STORY a scene to remember it by

B already stands high on the slope, and 還 walks one more step up past it.
字源記憶法
框 · Frame
[A] 比 [B] 還 [quality]
觸 · Trigger
B is already high on some scale, and A goes one step beyond it.
序 · The move
1Set up the comparison: A 比 B, with B already holding plenty of the quality.Is B's level taken for granted as high? 還 leans on that baseline.
2Drop 還 in just before the quality.還 sits before the adjective, not after B and not after the quality.
3End on the bare quality.No 很 inside the comparison; 還 already carries the boost.
例 · Examples
1這家餐廳this restaurant的水's watercompared to / than飲料drinkseven (more)expensive
At this restaurant the water is even more expensive than the drinks.
界 · Boundary
更 (A比B更X)
更 raises A above B flatly. 還 raises A while leaning on B's already-high level, stressing that A tops an already-high standard. Plain boost vs boost-past-a-high-baseline.
A比B X (no adverb)
Bare 比 just ranks A over B. Adding 還 keeps the ranking and stresses that A exceeds an already-high B. Ranking vs escalation.
這家餐廳的水比飲料很貴 → 這家餐廳的水比飲料還貴 (use 還 for the boost; 很 cannot sit inside a comparison)
這家餐廳的水還比飲料貴 → 這家餐廳的水比飲料還貴 (還 goes before the quality, after B)
這家餐廳的水比飲料還很貴 → 這家餐廳的水比飲料還貴 (drop 很; 還 already carries the degree)
English fuses the boost into the adjective ('even more expensive') up front; Chinese keeps the standard with 比 in the middle and slots 還 right before the bare quality.