Various grammatical rules and style guides determine which relative pronouns may be suitable in various situations, especially for formal settings.
The choice of relative pronoun typically depends on whether the antecedent is human or non-human: for example, who and its derivatives (whom, whoever, etc.—apart from whose) are generally restricted to human antecedents, while which and what and their derivatives refer in most cases to things, including animals.
Some writers and style guides recommend reserving that for non-human cases only, but this view does not reflect general use.
It conveys these meanings by deploying a non-restrictive relative clause and three short intonation curves, usually marked-off by commas.
Now the sentence means: it is the builder who builds "very fine" houses who will make a large profit.
It conveys this very different meaning by providing a restrictive relative clause and only one intonation curve, and no commas.
In the first example, for instance, there is no suggestion that the narrator has two fathers because the relative clause does not express a distinguishing property of the subject.
For clarity, we can look at the case of non-human antecedents using the previous example: Of the two, it is consensus that only which is commonly used in non-restrictive clauses.
[7] Equivalently, the two cases would be applied where the statements are logically: The dispute concerns restrictive clauses.
[11] It was championed in 1926 by H. W. Fowler, who said: "If writers would agree to regard that as the defining [restrictive] relative pronoun, and which as the non-defining, there would be much gain both in lucidity and in ease.
Such preposition-stranding is perfectly grammatical and has been used by the best writers for centuries, though it was, in the past, criticized by prescriptivist grammarians as being either ungrammatical or informal.
[15] In the case of the construction with a stranded preposition, however, the subjective form (e.g. "who") is commonly used, as in especially in informal style.
Use of the objective case with a stranded preposition, as in is somewhat rare, but occasionally found, even in informal style.
[17] Variations may be encountered in the spoken and informal English, but the most common distribution of the forms of pronouns in relative clauses follows: The word that, when used in the way described above, has been classified as a relative pronoun; however, according to some linguists it ought to be analyzed instead as a subordinating conjunction or relativizer.
According to Rodney Huddleston and Geoffrey Pullum, that is not a relative pronoun but a subordinator, and its analysis requires a relativized symbol R as in (The film that I needed [R] is not obtainable).
There are also similarities between the (purported) relative pronoun that and the ordinary conjunction that: the weak pronunciation /ðət/ is (almost invariably) used in both cases, and both of them are frequently omitted as implied.
In a 1990 article, Ellen Prince observed that such constructions were common in spoken English but are officially ungrammatical.
[20] For example: In this case, removing the underlined resumptive pronoun results in an acceptable gapped relative clause: In other cases, the resumptive pronoun is used to work around a syntactic constraint: In this example, the word it occurs as part of a wh-island.
Attempting to extract it gives an unacceptable result: Gapless relative clauses may also occur without a resumptive pronoun:[21]