what kind of verb is "MUST"?
He must be upstairs. / They must have gone to the store. /
'must' equals "With all the facts considered, the only logical conclusion to my mind is, he is upstairs / they have gone to the store.
It's also used to express social meanings of necessity/strong compulsion.
You must tell the police. / They must study or they'll fail. / He must report that as income.