Yesterday my niece had problems with the sentence pattern "must have V-en." Not understanding why "must" is followed by "have V-en," she asked, "Isn't "have V-en" a present perfect?" I told her it has nothing to do with the present perfect tense.
To understand "must + have V-en," one must know the pattern "must + V" first. "Must + V" is used to make a strong deduction about the present.
e.g. Jack is yawning. He must be tired.
The boy is trembling. He must feel scared.
Then what if you want to express strong deductions about the past? You cannot say "must + V-ed" because "must," a modal verb, is always followed by a bare infinitive. So you have to put "have V-en" after "must" to show it's a deduction about the past.
e.g. The ground is wet. It must have rained last night.
Jack is using Peter's cellphone to call his father. He must have left his at home.
A similar pattern: seem to have V-en
e.g. Teresa is slender now. She seems to have lost quite a few pounds.
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