Front a stative verb with 多 and close with 啊 to exclaim how far a quality reaches — how pretty, how big, how good it is.
框 · Frame
[subj] 多 [stative verb] 啊！
觸 · Trigger
A quality has struck you as far past ordinary and you want to throw it out as feeling, not ask for its measure.
序 · The move
1name the quality on a scale: 漂亮, 大, 好, 高is it a stative verb that can run up a scale?
2front it with 多 and leave the degree slot open: 多漂亮is there no 很/太 already filling the degree?
3close the line with 啊 and an exclamation mark: 多漂亮啊！does the 啊 turn it into a cry of feeling, not a request for a number?
例 · Examples
1他的his新new髮型hairstyle多how (to what a high degree)漂亮beautiful啊exclamation particle！
How beautiful his new hairstyle is!
界 · Boundary
多Vs（degree question）
The closing 啊 is the whole difference. 你哥哥多高？ asks for a number and waits; 他多高啊！ throws out how tall he is as a marvel. Same 多+Vs string — 啊 flips the probe into an exclamation.
很 states a quality flatly inside a sentence (他很漂亮 = he is pretty); 多…啊 cries it out with feeling (他多漂亮啊 = how pretty he is!). 很 fills the degree; 多…啊 leaves it open and lets the mouth carry it.
asking instead of exclaiming: 他的髮型多漂亮？ → 他的髮型多漂亮啊！ (drop the 啊 and it reverts to a degree question)
double-filling the degree: 他多很漂亮啊 → 他多漂亮啊 (多 holds the slot, no 很)
swapping in 嗎: 他多漂亮嗎 → 他多漂亮啊 (嗎 makes a yes-no question; 啊 makes the exclamation)
English fronts the whole how-phrase (how beautiful it is); Mandarin glues 多 to the stative verb in place and hangs 啊 on the tail — 多漂亮啊, no fronting and no inversion.