Modal: Must and Mustn't

must shows strong obligation; mustn't shows prohibition.

Examples

• You must wear a helmet.
• You mustn't smoke here.

Key rules

• must shows strong obligation, often from the speaker: You must wear a seatbelt
• mustn't means prohibition: You mustn't smoke here
• The form is the same for all subjects
• No to and no -s after must

Common mistakes

• He musts go → He must go
• You must to study → You must study