The past of "must"

Question

PUMPKIN HEAD (JOYCE CAROL OATES)

He asked Hadley how long she’d lived in the house, and when she told him that she and her husband had moved there in 1988 he maintained his pained, fixed smile but did not ask about her husband. He must know, then, she thought. Someone at the co-op has told him.

I know that the past of must is had to. The Oxford online dictionary says that must can be used as past tense in reported speech ("she said she must be going"). But the sentence in the text is not reported speech. The modal might would be grammatically correct in the sentence, for might is the past of may. But can I use must as the past of must?

And I'm confused by the last sentence when the author suddenly uses the present tense ("has told him"). Is there a connection with the previously used must?

Answer 1

He must know, then, she thought. Someone at the co-op has told him.

The two marked sentences are both direct "speech", although Oates does not mark them with quotation marks—probably because they are not spoken but only thought. The must here therefore represents present tense.

But the past tense of must is not had to, though had to may substitute for it. The past tense of must is must^†.

In any case, there is no justification for using either may or might here; either would completely change the meaning.

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^† Historically, in fact, must was the past tense of the verb mote, which long since disappeared from use. Must appears to be following in its footsteps, being more and more replaced by the more flexible quasi-modal HAVE to.