Prompted by What does 'should' mean in this sentence?, instead of asking what it means, I'm interested in what part of speech it is.
The sentence is:
She walked through the forest, and who should she see, but the Big Bad Wolf
Should is being used to mean did in this sentence, which is an unusual use for should.
The general form seems to be:
[Interrogative] should [event] but […]
(But is replaceable by except, only, etc.)
Is this a subjunctive? Is it a vestige from Old English?
Best Answer
This seems to be OED sense 17 for "shall":
Since the earliest citation is 1626, it does not seem to be a vestige from Old English.
The answer to "what is the part of speech?" is "modal".