Must in past. Obligation versus deduction

Question

If you can use "have" as a modifier to make modals past tense, why can't we apply them across the board. For example May/Might in past "you might have known." Or "He must have been here." These are perfectly acceptable examples, but when we come to obligatory usage the structure is more important.

For example: "Do you have to insult my mother every time you see here?" Is interchangeable with "Must you insult my mother every time you see her?"

Conversely why can we say "Did you have to insult my mother every time you saw here?" This use in the past is grammatically correct and conveys an action that used to happen, but no longer happens, for instance the mother in question died.

The main question is why can't we say "Must you have insulted my mother every time you saw her?" I am a native speaker and I would understand that question without thinking the speaker was using my language incorrectly. This is from the perspective that as long as language follows syntactic patterns there is room for correct usages that may not be codified in books, but still can be used to convey meaning.

Answer 1

The simplest answer is that we can't say "Must you have insulted..." for the same reasons that we can't say "Did you have insulted..." The form of the question makes it wrong, not any property of must.

Though it is often said that must is a defective modal with no past form, technically it has no past from because it is already a past form; the past form of mote. Mote is no longer used, except possibly in rituals, and must has taken its place.

Adding to the complication, must also has two forms of modality. Deontic used on its own, as in "You must buy a new car" indicating that in the ideal world your car would be new, and epistemic when used with have as in "he must have bought a new car" based on the evidence of a shiny new car in his drive. This can also be viewed as must for obligation and must for speculation.

Applying this to the question, on top of the issue of the question form, as you want to ask about past events there is no modality involved. It is not speculating about insults to your mother, nor laying an obligation on anyone to insult your mother in the past.

Answer 2

I agree with your judgment about the example of your main question -- it's no good. However, I think its declarative counterpart is okay.

*"Must you have insulted my mother every time you saw her?"
"You must have insulted my mother every time you saw her."

The problem does not seem to be with the question form, specifically, because we see it below in a declarative sentence that has subject auxiliary inversion:

*"Never must you have left early.
"You must never have left early.

I don't understand the restriction that prevents inverting "must" in these cases. It isn't general to the modal verbs, since other modals sound okay when inverted. It is triggered by the perfect "have" but may also be triggered by some stative verbs.

Answer 3

The past of "must" is also must. If English would use "must" as form of present tense and also of past tense this would lead to an annoying ambiguity. So it is a wise rule not to use "must" as past tense and to use the substitute with to have to do.

In special cases you can find must as past tense in subordinate clauses.