RELATIVE PRONOUNS: RESTRICTIVE AND NON-RESTRICTIVE
The relative pronouns are: who, whom, whose, which, and that. Relative pronouns introduce subordinate clauses functioning as adjectives. Use commas to set off nonrestrictive subordinate clauses, and do not use commas to set off restrictive clauses. The choice of relative pronouns is determined by the way the pronoun is used and the noun or pronoun to which it refers. Who, which, and that take verbs that agree with their antecedents.
1. Relative pronouns introduce subordinate clauses functioning as adjectives.
The man who robbed us was never caught.
The arrow that has left the bow never returns.
In addition to introducing the clause, the relative pronoun, in this case
who, points back to a noun or pronoun that the clause modifies (man). In the second sentence, that points back to arrow.
Relative pronouns are sometimes "understood."
The things [that] we know best are the things [that] we haven't been taught.
2. Use commas to set off nonrestrictive elements. Do not use commas to set off restrictive elements.
A restrictive element defines or limits the meaning of the word it modifies and is therefore essential to the meaning of the sentence. Because it contains essential information, a restrictive element is not set off with commas.
Restrictive: For camp the children need clothes that are washable.
If you remove a restrictive element from a sentence, the meaning changes significantly, becoming more general than you intended. The writer of the example sentence does not mean that the children need clothes in general. The intended meaning is more limited: The children need washable clothes.
Nonrestrictive: For camp the children need sturdy shoes, which are expensive.
A nonrestrictive element describes a noun or pronoun whose meaning has already been clearly defined or limited. Because it contains nonessential or parenthetical information, a nonrestrictive element is set off with commas. If you remove a nonrestrictive element from a sentence, the meaning does not change significantly. The children need sturdy shoes, and these happen to be expensive.
3. The choice of that, which, who, or whom is dependent upon the way in which the pronoun is used within the sentence, as well as the noun or pronoun to which it refers.
In general, use that with restrictive clauses and which with nonrestrictive clauses.
Do not use which to refer to persons. Use who instead. That, though generally used to refer to things, may be used to refer to a group or class of people.
Fans wondered how an old man who (not that or which) walked with a limp could play football.
The team that scores the most points in this game will win the tournament.
Who is used for subjects and subject complements; whom is used for objects.
Who is bringing what for whom?
4. Who, which, and that take verbs that agree with their antecedents.
Take a suit that travels well. (A suit... travels well.)
Take clothes that are easy to wash. (Clothes... are easy to wash.)
Sometimes problems occur when one of the and only one of the are used. Generally, one of the is treated as plural, and only one of the is treated as singular.
Our ability to use language is one of the things that set us apart from animals.
The antecedent of that is things, not one. Several things set us apart from animals, and language is one of them.
Carmen is the only one of the applicants who has the ability to step into this position.
The antecedent of who is one, not applicants. Only one applicant, Carmen, has the ability to step into the position.