In the CHILDES lab, you’re looking for non-subject wh-questions mostly, which means you need to be able to distinguish those wh-phrases that are subjects and those that are not.

For a verb like “eat” it’s pretty straightforward. The subject does the eating, the object gets eaten.

When the verb is “to be” however, it becomes much less clear. In the lab writeup, I suggested that “what’s that?” should count as a case where the wh-word is not a subject. Here’s my thinking on that.

The easiest way to think about this I believe is to try “unrolling” the question into a statement with a word like “something” or “someone” in it. So, for example: “That is something—what?” This is the form I had in mind when I said that “what” here is not a subject, but an object (and “that” is the subject). That’s because the other option, “Something is that—what?” sounds pretty off to me. To the extent that “to be” means something like “equals” there is still room for debate, but here I think the contrast is pretty clear between “That is something” and “Something is that.”

With “to be” it is generally possible to invert things in either order, but I find that one of them sounds kind of straightforward and natural, and the other sounds like poetry or something, stylized. Still English, but not the basic form.

Another example, then, would be “Which one [is] on my red [box]?” Is “Which one” a subject here? What I would compare is “Something is on my red box—which one?” and “On my red box is something—which one?” I find the first of these much more natural. (You also have to assume that it’s clear what “which one” is supposed to mean in the context, but abstracting away from that, the order “This is on my red box” is much more natural than “On my red box is this.”) So, since the first of those was more natural, and in that one “something” is the subject, I would say that “Which one” in this case is a subject. Meaning that this is a subject wh-question, and actually not one that we want to include in our computations, since we’re looking for non-subject-wh-questions.