务必 is an adverb meaning 'must' or 'certainly', used to express strong necessity or urgency. It combines 务 (wù), meaning 'to attend to' or 'affair', and 必 (bì), meaning 'must' or 'inevitable'. Together, they emphasize that something is absolutely required — not just recommended or advisable, but non-negotiable. The word carries formal, authoritative weight and is common in instructions, official notices, reminders, and polite yet firm requests.

务必要 be placed before the verb it modifies and cannot stand alone as a sentence. It’s more emphatic than 必须 and slightly more formal than 一定 (when used for obligation). While it can appear in spoken Chinese, it’s especially frequent in written contexts like emails, announcements, or safety guidelines — for example, '务必戴口罩' (You must wear a mask) conveys higher urgency than '要戴口罩' or '应该戴口罩'.