"Must" past tense

Previous post... what is the past tense of "must"? "Had to?" (That is what one source says, though I suspect that is an Americanism.) "Must have"? The sentence I had was complicated because it was in the negative (She must not have expected him/she must not expect him), and so it complicated the verb. Anyway, here is an interesting discussion of this issue-- is "must" its own past tense? It's a bit archaic, maybe, but "must" has been the past tense of "must," and one respondent in that discussion points out that modals (the little verb-words that add a condition to a verb, like could or should) often do dual duty, with present or past tense determined by context or some other indication (like "five years ago"). Interesting, anyway. The respondent mentions that the OED makes a distinction for "indirect speech" (like "He told her that he must go"). I think a character's internal thoughts might count as indirect speech, where there's actually a certain existential confusion of past and present. (Notice how we debate whether a thought should be in present or past tense-- same issue.)