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)
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It’s still acting as a modal verb here; this is an ellipsis for
I must [go] to England.
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".