What did "must" mean when used as a non-modal verb (sorry, I don't know the technical term) in Early Modern English? For example:
I must to England; you know that?
(Hamlet, Act III, Scene IV)
It’s still acting as a modal verb here; this is an ellipsis for
The OED glosses this usage, under the main sense of “expressing necessity,” as “(b) With verb of motion understood. Now arch.”.
(Must does also have some non-modal usages: “to become mouldy, musty, or mildewed”; “to dress or dust with hair-powder”; “of a male elephant, etc.: to come into a state of musth”. But I suspect none of these is what you, or Hamlet, meant.)
Edit: an earlier version of this answer described this instead as the OED’s “(c) with implied infinitive taken from the context,” which arguably fits, but I think the more specific (b) is probably more apposite here.
Shakespeare also wrote (perhaps originated) the expression "must away", which is the same usage. I had thought that it was a shortening of "must hie", which I had believed was a common expression in early modern English, but a quick search reveals only Emerson and Burns.
In any case, as PLL stated, it is still acting as a modal verb, and the implied main verb is a verb of motion, such as "go" or "hie".