Must

Must is a modal verb used to show obligation.  Like all modal verbs, must is followed by a base verb:

MUST        +        BASE VERB

Examples:

• He must go to class today.
  
• They must work tomorrow.
  

Must is also used to show possibility, and is used in situations where the speaker is very certain.

Examples:

• He ate 3 pieces of cake.  He must like cake.
  
• They worked for 12 hours today.  They must be tired.