In a question like "Who hears a noise?", is the subject of the sentence who?
I can think of a few tests for subjects like: "the subject is the phrase that inverts with the auxiliary to form a question". But this is a question and there is no auxiliary. We could say that's because it's a subject, but that would be just to presuppose that it's a subject in the first place.
I can think of tests like: "the form of the verb will agree with the number of the subject", but on the other hand who does not seem to have fixed grammatical number, and it sometimes seems like who might actually be agreeing with noun phrases later in the sentence:
- Who are the most prolific writers of our age?
- Who is the most prolific writer of our age?
There are also tests like "the subject is usually the first noun phrase in the sentence". But of course if we have a question like:
- Who have you bitten?
... you, not who seems to be the subject. The first noun phrase test is not very good.
I also know that some theories of grammar say that there is a gap in wh- questions that the wh- word is extracted from. So can this gap for instance be the real object of a question? If so, is there a gap in questions like mine functioning as subject?[See community wiki post in the linked to question]
How can we show whether who is really the subject of my original example question? And how about other wh- words like what? Can they (also) function as subjects?