Be sure to / by all means do not (务必 / 切勿)

Structure 务必 + required action （positive） · 切勿 / 切莫 + forbidden action （negative）

务必 (wùbì) and 切勿 (qièwù) are the emphatic imperatives of notices, instructions and formal announcements. 务必 commands one to do something "without fail, no excuses": 请务必准时到场 = "please be sure to arrive on time". Its negative is 切勿 (or the even more literary 切莫): an exhortation NOT to do something, "by all means do not…, on no account…": 切勿轻信陌生人 = "on no account trust strangers lightly". Against the everyday 别 / 不要 (HSK 1–2) of speech, 切勿/切莫 belong to the written language of signs, leaflets and rules. 勿 is the classical negator of prohibition, inherited from literary Chinese.

Examples

会huì议yì十shí分fēn重zhòng要yào，请qǐng各gè位wèi务wù必bì准zhǔn时shí出chū席xí。
The meeting is very important; everyone is asked to be sure to attend on time.
操cāo作zuò前qián务wù必bì仔zǐ细xì阅yuè读dú说shuō明míng书shū。
Before operating it, be sure to read the manual carefully.
切qiè勿wù轻qīng信xìn陌mò生shēng人rén发fā来lái的de链liàn接jiē。
On no account trust links sent by strangers.
药yào品pǐn须xū放fàng在zài儿ér童tóng接jiē触chù不bù到dào之zhī处chù，切qiè勿wù误wù食shí。
Medicines must be kept out of children's reach; by no means swallow them by mistake.

Levels are indicative: they follow the standard Chinese Grammar Wiki (AllSet Learning) progression, not an official HSK 3.0 mapping. We never assign levels by eye.

This point and the other 370, with audio on every example, inside Wanyu.