Must remains unchanged in form whatever be the number and person of the subject. It can point to the present or future. It can refer to the past only when it is used with the present perfect tense of the principal verb.

• He must have reached home. (Past)
• He must go now. (Present)
• You must file a petition. (Future)
• They must pay damages. (Future)

Uses

Must expresses compulsion or strong obligation. It is stronger than should.

• He must obey my instructions.
• They must act now.
• I must go.

Probability
Must can express probability or likelihood.

• He must be mad to do this.
• That must have been a shooting star.
• ‘There is the door bell.’ ‘That must be the post man.’

Related posts:

1. Modal Auxiliary Verb Will
2. Uses of Should
3. Modal auxiliary verb shall
4. Uses of Would
5. What are Auxiliary verbs?