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A student has asked me to give him a very clear explanation of why we use two structures in the past tense to make questions:
Did (eg Did you see him yesterday?) Were (were you with him yesterday?)
I find that when I try to explain I go round in circles and confuse him even more. He's pre-intermediate and needs a very simple explanation that he can grasp and my brain is fried from trying...any help would be gratefully received! Thanks :)
This is a matter of what linguists call do-support.
The inversion which we employ in yes/no questions like "Did you see him?" and Were you with him?† is subject-auxiliary inversion—that is, the subject inverts only with auxiliary ('helping') verbs: have, be, do and the modals can/could, may/might, must, shall/should and will/would.
If the declarative version of your sentence has an auxiliary, you invert it with the subject to make a question:
SUBJYou AUXhave seen him → AUXHave SUBJyou seen him?
But lexical ('main') verbs, like see, which can stand alone without an auxiliary, don't invert;‡ so if the declarative version uses one of these verbs without an auxiliary, how do you make a question? —You bring do in to act as the auxiliary you need, and invert that with the subject:
SUBJYou saw him → SUBJYou AUXdid see him → AUXDid SUBJyou see him?
There's just one tricky thing here. Have, do and be can all act as either auxiliaries OR lexical verbs. When you use do by itself as a lexical verb, as in do your homework, it requires do-support for inversion in questions:
Did you do your homework?
But be always acts as an auxiliary and inverts without do-support, even when it's the only verb in the clause:
SUBJYou AUXwere with him → AUXWere SUBJyou with him?
And lexical have is betwixt-and-between: in ordinary speech today it requires do-support in questions:
Do you have any questions?
But for a long time lexical have could be used like an auxiliary, and there are still a few people who use it that way:
Have you any questions?
† and in negations like I don't see him—but we won't worry about that here.
‡ Once upon a time they did invert—Saw you the field? Came you from Shrewsbury?—but that use mostly disappeared some 400 years ago, with a couple of exceptions addressed later.
For most verbs, the simple past tense when making questions is formed by beginning your sentence with did, then comes the subject, and finally the verb itself in present tense form. to be is just somewhat special in that regard. You simply put the verb (was or were) in front of the subject. That's really all there is to it at the end of the day.
