The lack of agreement about the correct word-class labels for certain expressions in this thread reflects the lack of agreement among academic grammarians.
Point taken, Piscean, but sometimes different labels reflect different
analyses.
My own interest in this topic took root a couple of years ago when a fellow ESL teacher -- one with a great deal more experience than I have -- asked me whether I could explain why "to" was optional in a sentence like
London would be a good place to go (to). My fancy academic hypothesis at the time was that the version without "to" derives from a deep structure with "where," "where" being obligatorily silenced upon being moved to the the front of the nonfinite (infinitival) relative clause modifying "place" (
*London would be a good place where to go) just as we must silence "which" in nonfinite relatives (
*London would be a good place which to go to) unless it functions as the object of a preposition and that preposition precedes it (
London would be a good place to which to go).
I consulted a former syntax professor of mine as to whether she thought my explanation was correct and what she thought. That was when I learned of the article I cited in my first post -- a masterful study that I am not competent enough to explicate for the benefit of learners. But the point that drew me to that sixty-page article in the first place was that Bresnan and Grimshaw were the first linguists to observe that "where" and "when" have two different syntactic categories: NP or PP.
Perhaps I need to work on finding ways of making the idea more learner-friendly before actually trying to use it to help learners; however, flawed as my abilities may be at present in that regard, it seems to me that the basic analytical point here is potentially immensely helpful in ESL pedagogy. Observant learners sometimes want to know why "to" is optional in sentences such as the one above, or in questions like
Where did she fly (to)? Why is it not the case that it must be one way or the other? It's because "where" can function either as the object of that preposition (a pro-NP) within the prepositional phrase, or as the prepositional phrase entire (a pro-PP).
It's the structural difference that matters to me here, not the labels.
Respectfully,
David