TSUMUGU · TBCL 5 (est.) · 語法
（請）勿
grammar point · tier 1 · formal prohibition 勿 (do not / must not)
· (qǐng) wù
Bars an action in formal or written notice: do not, must not; 請 prepended softens the ban to a courteous request.
Hook inherited from 不.
Bars an action in formal or written notice
框 · Frame
（請）勿 [verb phrase]
觸 · Trigger
Posting or reading a formal prohibition on a sign or notice — bar an action without sounding like a personal scolding.
序 · The move
1Take the action you are barring as a bare verb phrase.Is this a notice or formal register, not casual speech? 勿 lives on signs, not in conversation.
2Set 勿 directly before that verb phrase.Nothing sits between 勿 and the verb — no 不, no object first.
3Prepend 請 to soften the ban into a courteous request.Drop 請 and the ban turns curt; keep it for public courtesy.
例 · Examples
1請please勿do not在in / at公共區域public areas吸煙smoke！
Please do not smoke in public areas!
界 · Boundary
不不 negates a fact, will, or quality (he does not smoke); 勿 issues a ban on the action (do not smoke).
別別 is the spoken don't, said to a person in front of you; 勿 is the written don't, posted on a notice.
✗ 勿不吸煙 → ✓ 請勿吸煙 (no second negator; 勿 carries the whole ban)
✗ 你勿吸煙了 → ✓ 你別吸煙了 (talking to a person face to face takes 別, not the formal 勿)
✗ 請勿吸煙嗎 → ✓ 請勿吸煙 (a prohibition is not a yes-no question; drop 嗎)
English 'do not' is one word in every register, so learners reach for 勿 in casual speech; 勿 is notice-board register, and a spoken don't wants 別.