（CGEL, Quirk et al, 1985, Sec 17.14）
Could someone please explain to me why the authors marked the other two relative words as wrong, especially "that". What is the underlying theory? I have found many examples with the same pattern using "that".
Hi, everybody—Concerning "She is the perfect accountant which her predecessor was not," it seems to me that the use of "which" instead of "that" guards against an absurd reading that would be possible with "that"—viz.:
A: Both accountants are perfect. Which one is Sarah?
B: She is the perfect accountant that her predessessor was not.
A: But her predecessor isn't there. I know that Sarah isn't Sarah's predecessor! Moreover, it's obvious that Sarah's predecessor is not Sarah!
Note that "who" is also starred as ungrammatical in Quirk et al. for that example. The absurd reading above is the same reading that the relative clause would yield with "who."
If you turn to page 1248 in Quirk et al., you will see that the "C" denotes that the relative pronoun is deemed by the authors to function as complement within the relative clause.
That is, the relative pronoun is not supposed to have the status of noun phrase here. If it did have the status of noun phrase, then "who" would need to be correct, insofar as an "accountant" (the would-be antecedent) is human.
But the relative pronoun has the status of complement. Consider the sentence "She is the perfect accountant." That is a sentence of praise. It is rather like saying "She is a perfect accountant" or "She is perfect as an accountant."
If the sentence had "a perfect accountant," the relative clause would be nonrestrictive: "She is a perfect accountant, which her predessessor was not." How, then, should we make sense of the sentence with "the"?
"She is the perfect accountant" picks out a certain ideal in the context. Indeed, her job may not even be that of accountant. The speaker has in mind an ideal fulfilled by the referent of "she" but not by her predessessor.
Regarding the second example ("This is not the type of modern house which my own is"), the zero relative ("This is not the type of modern house my own is") sounds very awkward to me. But why have they starred it and "that"?
Here's how I'm inclined to defend them. Underlying a sentence like "This house is not that type" is a concealed "of" ("This house is not of that type"). No house is a type (an abstract thing). But houses can be of various types.
Thus, I'd say that the sentence "This is not the type of modern house which my own is" may be said to be short for "This is not of the type of modern house of which my own is." But I admit that I am reaching far for a defense here.
Long live Quirk et al.'s A Comprehensive Grammar of the English Language!