Form
Subject + must + Verb
The lights are on. He must be home.
You haven’t eaten all day. You must be hungry.
Meanings
Positive Deduction (~95% certain)
We use must when we are almost certain that something is true because we have strong evidence or reasons.
Evidence: All the lights in his house are on.
Deduction: He must be home.
Important: This is different from must for obligation. Here, must is for a logical conclusion.
Deduction: You must be tired after your long journey.
Obligation: You must finish your homework.