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The auxiliary verb must can express:

•  Obligation, duty, or need to do something:
I must attend this meeting. I have to, I need to attend this meeting.
He must pay his taxes before the end of the month. He is obliged to pay his taxes before the end of the month.
•  certainty or a logical conclusion:
It's late, he must be at home now. It's late, so I imagine he is probably at home now.
The hardware works fine, it must be a software problem. The hardware is working perfectly, so I imagine it's probably a software problem.
In its negative form, mustn't always expresses prohibition:
You mustn't talk to strangers, kid. It's forbidden to talk to strangers, kid.

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