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2.2.1.2.10 must
2.2.1.2.10.1 Expressing insistence. For example:
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You must not smoke here!
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The doctor insists that it's dangerous and it must be treated immediately.
2.2.1.2.10.2 Expressing an opinion about something that is logically very likely. For example:
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The right to join a party must logically include the right not to do so.
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It must be terrible for a person that has never smoked. It's stuffy here!
2.2.1.2.10.3 The past tense of must
In a forum, someone said etymologically, must is the past tense of mote (as in "so mote it be"), so one wouldn't expect it to have a past tense itself. And there are examples of must itself being used in the past tense, as “What he did, he did because he must.” (Ralph Waldo Emerson, Essays, First Series, 4. “Spiritual Laws.”)
These days we often use had to as a past tense of must. In an article, Ku Tak Ming explains the usages as follows:
“When we use must to mean ‘be obliged to,’ the past tense of must is had to. If we use must to mean ‘certainty,’ the past tense is must have. For example:”
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I must pay the bill now. --> I had to pay the bill then.
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We must be near a village (now). --> We must have been near a village (then).
“However, when must is used after verbs like said, told, knew, felt, etc., indicating that one ‘has to’ or ‘must’ do something at an earlier time, then must can also be used in the past tense to refer to action in the past. For example:”
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He told me I must pay the bill.
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We felt we must be near a village.