务必【wùbì】

务必 goes before a verb and presses the listener to do it without fail. It looks ahead to something not yet done and carries the weight of an instruction, softened by 请 when it's a request. 要 often follows it. It's formal: notices, emails, announcements, and requests made in earnest.

务必 or 必须

必须 states a requirement and can describe anyone. 务必 is a request or order aimed at the listener, or at whoever a notice addresses.

The negative

For "be sure not to", put 不要 after 务必. 务必 doesn't take a bare 不, and there's no 不务必 – "don't have to" is 不必 or 不用.

• × 务必不迟到 → 务必不要迟到
• × 你不务必来 → 你不必来