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Look at this sentence:
Kathy's office is dark, the coffee on her desk is cold, and her answering machine shows seven unheard messages. Kathy must not have come in to the office yet.
This usage of "must" is what we call "logical inference" or "logical conclusion."
When we draw a conclusion based on information, we can use must.
For example, if Sally has left the house, but the radio is still on, we can say "She must have forgotten."
If we see somebody who has just come back from a long day at work with no break, we can say "You must be tired."
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