Aha! This is one of those traps where the English language stops making sense.
When you're asking "What [thing]..?" you're actually either asking "Which [thing]..?" ("What/which apple did you take?") or "What kind of [thing]..?" ("What (kind of) music do you listen to?"), both of which would require a different question word in Hungarian (melyik and milyen, respectively). So by asking "What example is this?", you imply that there is a group of examples which you pick one item from.
"What is this example?" doesn't care about a group, but shows genuine interest in the actual content of the example. It's a small difference, but an important one when dealing with foreign grammar.
I see exactly what you mean. But I just don't think "what is this example" sounds very natural in English really. Example as a word precludes that you are looking for something specific anyway. "What is this an example of?" Argh. Trying to get Hungarian into English and vice versa is a painful process :P
Yeah, the sentence doesn't make a lot of sense in either language. Maybe something like "What is that sound?" would be better. (Mi az a zaj?)
My point here is just if you start to translate it as "What example is this?", you might be tempted to write "Mi példa ez?", which really doesn't work.
I think I can come up with two scenarios.
A student joins a group that's learning about, say, psychology from a book. She sees a drawing in there that seemingly has nothing to do with the subject at hand, so she confusedly asks "What is this example?"
A physics teacher starts a calculation: "Suppose Superman flies at 100 km/h straight upwards, holding a bowling ball with a mass of 10 kilograms. 5 minutes after starting off the ground, he lets it go, so we..."
And the tired pupils say "What (the hell) is this example now?"
This is a so-called nominal or copula sentence where both subject and object (if those terms are even applicable) are nominative. The most prominent case is with using the verb to be in English, and van in Hungarian, accordingly.
Though the English language is a bit lax on that. See "I am the man" vs. "The man is me", which I deem is more colloquial than "The man is I".
Yeah, arguably the English case distinction (in modern usage) has ceased to be nominative versus accusative, with the nominative case basically only being used when preceding a verb and the accusative being used when following a verb or as a standalone. Who wants ice cream? Me. I do.
Both are in nominative, and the question has no object. It's basically a pronoun = noun structure, like "He is a boy." / "Ő egy fiú." The pronoun ő is the subject, while the noun egy fiú is basically the predicate of the sentence, and both take the nominative. In Duo's sentence the only difference is that the pronoun is an interrogative pronoun.