Now, referring to the sentence, "The fire burned two hundred acres of forest yesterday," when I was naming the various questions the crud answers, you may have noticed I didn’t mention “of forest.”  That’s because it answers “of what,” which could easily be confused with answering the WHAT happened (the verb.)  “Of what” is a prepositional phrase.  The "what" follows a preposition

One grammatical rule is that a subject can never be the object of a preposition.  (Subjects and objects occupy two completely different positions in the sentence.  They are like opposites.)  So keep that in mind when you are trying to decide what the subject is in sentences that have prepositions in them.

  • The stack of books fell down.  (Stack is the subject, not books.)

  • The list of my purchases has disappeared.  (The list has disappeared, not the purchases.)

  • The shelves in the closet need painting.  (The shelves need painting, not the closet.)

  • Balls from the nearby tennis court often hit my window.  (The balls hit my window, not the tennis court.)

Therefore, whatever follows the preposition cannot tell WHO, as in who (or what) did it.  (As you can see, the subject of the above sentences are NOT the books, the purchases, the closet, or the tennis court.)  Nor can it tell WHAT, as in what happened.    

As it turns out, "of forest" ends up being just description -- crud.  "Acres of forest" is the equivalent of saying "forested acres."  In the same way, phrases like “for what” equals why, “in what” equals where or how, “to what” equals why or where, "at what" equals where – all those must be crud.  None can be the subject or the verb.