The treatment in Quirk et al.* of relative clauses in which the head is a (nonprepositional) subject complement is, to my perception, confusing. (see the final part of this posting for explanation of "nonprepositional subject complement.")
In my posting of Sept. 21, referring to the explanation and examples on p. 1260 of Quirk, I said
"The authors give no examples of restrictive relative clauses in such a context, and I can't think of any restrictive relative clause in that kind of context that is limited to "which" as a relative pronoun. "
(We have to remember that the kind of relative clause we are talking about is one in which the clause characterizes (attributes qualities to) the subject complement, not one in which the clause identifies it.)
Looking further through the book, I came across a couple of examples of restrictive relative clauses in which the head word is a subject complement and which the authors say allow only "which" as relativizer:
"” She is the perfect accountant which/*who/*that her predecessor was not
"” This is not the type of modern house which/*that/*( ) my own is (pp. 1248-49)
These relative clauses also characterize the subject complements of the main clauses, so the sentences seem very similar to the one on p. 367 with "that."
These two examples are consistent with the authors' statement on p. 1260 that both nonrestrictive and restrictive relative clauses in which the head is a subject complement take only the relativizer "which" and not "that."
When I read these examples, however, it seemed to me that the appropriate relativizer was "that," not "which." The authors apparently do not agree, but I can't see why. The only natural relativizer seems to me to be "that."
Now, there is ONE difference between the rule and example on p. 367 of the same book that uses "that":
"” She's not the brilliant dancer (that) she used to be.
...and the statement on p. 1260 that says that the only correct relativizer in a restrictive relative clause that has a subject complement as head is "which."
The difference is that in the example on p. 367 with "that," the subject complement in the main clause and the one in the relative clause are identical in reference, while in the examples with "which" on pp. 1248-49 the two are different. But why that fact should make a difference in the choice of relativizer I don't know.
This may, however, be the distinction that makes the authors prescribe "that" in one case and "which" in the other. The authors don't say anything about "same" vs. different noun phrases. And I still believe that the complementizer should be "that," not "which" in all restrictive relative clauses modifying subject complements, as long as the relative clause is one of characterization.
To answer Gisele's first question: A prepositional complement is not a prepositional phrase. A prepositional phrase can be a subject complement, as in Gisele's example, "The child was in tears." But that's not the same as a prepositional complement. A prepositional phrase that is a subject complement is an adverbial.
The kind of "prepositional complement" that Quirk et al. were referring to on p. 1260 is the kind with a determiner as head, such as
"” There are several schools here, all/some of which I can recommend
This kind of prepositional complement is not related to the present question.
The mystery surrounding restrictive relative clauses that modify a subject complement continues. If anyone has a suggestion as to why "that" is preferred in one restrictive relative clause modifying a subject complement and "which" in another, please enlighten us all!
*A Comprehensive Grammar of the English Language (Longman, 1985)