Relative clauses in the English language are formed principally by means of relative words. The basic relative pronouns are who, which, and that; who also has the derived forms whom and whose. Various grammatical rules and style guides determine which relative pronouns may be suitable in various situations, especially for formal settings. In some cases the relative pronoun may be omitted and merely implied ("This is the man [that] I saw", or "This is the putter he wins with").
English also uses free relative clauses, which have no antecedent and can be formed with the pronouns such as what ("I like what you've done"), and who and whoever.
Modern guides to English say that the relative pronoun should take the case (subject or object) which is appropriate to the relative clause, not the function performed by that clause within an external clause.
Overview The basic grammatical rules for the formation of relative clauses in English are given here.These rules refer to actual usage, as described in standard books on grammar, such as and Some prescriptivist style guides, such as propose additional rules concerning which relative pronouns should be used in which circumstances. More details can be found in the article on who.
The basic relative pronouns are considered to be who, which and that, but an alternative analysis of that as a relativizer is presented in a succeeding section. The relative pronoun comes at the very start of the relative clause unless it is preceded by a fronted preposition: "The bed on which I was lying". (It is normal to slide the preposition to the end of the clause and leave it stranded, or dangling: "The bed which I was lying on"). The relative clause may start with a larger phrase containing the relative pronoun after a preposition: "The bed, the owner of which we had seen previously, ...", or "The bed, lying on which was a small cat, ..." who is used only with its antecedent referring to a person ("The man who ..."); which, referring to a thing ("The flowers which ..."); that, referring to either a person or thing ("The woman that ...", or "The flowers that ..."). that is used only in restrictive relative clauses, and is not preceded by a comma ("The teacher that looks worn-out", or "The car that looks worn-out"); but who and which may be used in both restrictive and non-restrictive clauses, and may or may not take a comma ("The teacher who looks worn-out", or "My teacher, who ..."), and ("The car which looks worn-out", or "My car, which ..."). In some styles of formal English, particularly American, using which in restrictive clauses is avoided where possible (see that or which below). whom is used only when its antecedent is the object of the relative clause, but not when its antecedent is the subject of the relative clause ("The officer nabbed the thief whom I saw")antecedent thief is the object of the relative clause; but not (*The officer nabbed the thief whom saw me)here the antecedent thief is the subject of the relative clause ("... the thief _ saw me"); who is correct here. When a preposition in the relative clause is placed in front (fronted), only whom or which is used ("The waiter to whom I spoke", or "The putter with which she wins"); it would never be acceptable to use who (*The waiter to who I spoke) or that (*The putter with that she wins). With informal style, the preposition is often dangled (or stranded), not fronted, so who and that may also be used (“The mailman who I spoke to”, “The mailman that I spoke to”, as well as “The mailman whom ...”); and (“The putter that she wins with”, or “The putter which ...”), or the zero relative pronoun is frequently used (“The putter she wins with”). (See Zero relative pronoun). When that is used in a restrictive relative clause and it is not the subject of the relative clause, it may be omitted entirely. For example: ("The dentist that I saw" or "The dentist that I spoke to") may be rendered simply ("The dentist I saw" or "The dentist I spoke to"). But any relative pronoun when used in a non-restrictive relative clause must not be omitted ("My dentist, whom I saw", or "My dentist, who spoke to me"); nor when its preposition is fronted ("The dentist to whom I spoke"); nor when its antecedent is the subject of the relative clause ("The dentist that saw me”, or “The dentist who saw me"). The verb in a relative clause takes the same person (first, second, or third) and number (singular or plural) as that of the antecedent of the relative pronoun. In ("The people who were present ...") the antecedent of who is people (third person, plural), so the verb to be takes its form (were) for third person and plural number; in ("I, who am normally very tolerant, ...") who‘s antecedent is the pronoun I (first person, singular), so the verb to be takes its form (am) for first person and singular number. whose indicates that the antecedent has a possessive role in the relative clause ("The man whose daughter I married"). Unlike who, it can refer to things as well as persons ("I found a car whose battery was dead"). Though there is some reluctance to use whose with a non-personal antecedent, such use is not uncommon and is perfectly grammatical. Whose is used in both restrictive and non-restrictive clauses (“The woman whose brother was recently married ...”, or "Sally, whose brother ...") and with both fronted and stranded prepositions ("The student in whose car we arrived ...", "The student whose car we arrived in ...") or larger phrases with a preposition ("My tutor, some of whose lessons..."). A relative clause whose antecedent is a whole propositionthat is, a matter (or person or thing) to be dealt withis formed with which ("The cake was burnt, which made me angry"); here which refers to the whole circumstance of the cake's being burnt. A formal, though uncommon, use of which is its being a relative determiner in non-restrictive clauses ("He painted a picture of the house, which painting I later destroyed"). Here, which may refer to persons as well as things (“Yesterday, I met three men with long beards, which men I remember vividly”). Which can also refer to the whole clause, followed by a word that represents the ideas of the clause ("Yesterday, I met three men with long beards, which meetings I remember vividly"). A preposition may be fronted in front of the relative determiner which ("Every day, he visits me at the arcade, from which fact I derive much pleasure"), as may a larger phrase containing a preposition ("He went to the park and the shopping center, both of which places ..."). A free relative clause has no antecedent and takes the role of an argument in the main clause. When referring to people, it is formed with the pronouns who, whom or whoever, whomever ("I'll take who you choose", or "I'll take whom you choose", or "I'll take whoever (or whomever) you choose"). When referring to things, it is formed with the pronouns what or whatever ("What I said annoyed her") where what stands for "the thing which ..." or "that which ...". Whichever is used when referring to people or things from a known set. (These are all called compound relative pronouns.) Also, there are the determiner (adjectival) equivalents which or what, or more usually, whichever or whatever ("I'll take whichever dish you choose", or "I'll take whatever dish you choose").
The words used as relative pronouns have other uses in English grammar: that can be a demonstrative or a conjunction, while which, what, who, whom and whose can be interrogatives. For other uses of whoever etc., see -ever.
Variables in the basic relative clause
Human or non-human antecedents 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.
The relative pronoun that is used with both human and non-human antecedents. Some writers and style guides recommend reserving that for non-human cases only, but this view does not reflect general use. Counter-examples can be found in literature: Shakespeare (the man that hath no music in himself, in The Merchant of Venice), Mark Twain (The Man that Corrupted Hadleyburg), and Ira Gershwin (The Man that Got Away); and informal English, especially speech, follows an actual practice (in using that and which) that is more natural than prescriptivist.
The possessive form whose is necessarily used with non-human as well as human antecedents because no possessive forms exist for which or that. Otherwise, to avoid, for example, using whose in "...the car whose engine blew up.." would require a periphrastic phrasing, such as "...the car the engine of which blew up", or "...the car of which the engine blew up".
English also makes the distinction between human vs. thing in personal pronouns (he, she vs. it) and certain other pronouns (such as someone, somebody vs. something); but some particular thingssuch a navy ships and marine vesselsare described with female pronouns, and pets and other animals are frequently addressed in terms of their gender or their (anthropomorphic) ‘personhood’. Typically, it is when these things-as-human become antecedents to relative clauses that their relative pronouns tend to revert to that or whichfor thingsrather than taking the regular who, whom, etc., for human referents. See Gender in English.
Restrictive or non-restrictive relative clauses The distinction between restrictive, or integrated, relative clauses and non-restrictive, or supplementary, relative clauses in English is made both in speaking (through prosody), and in writing (through punctuation): a non-restrictive relative clause is surrounded by pauses in speech and usually by commas in writing, whereas a restrictive clause is not. Compare the following sentences, which have quite different meanings and intonation, depending on whether the commas are inserted:
(1) The builder, who erects very fine houses, will make a large profit. (non-restrictive) (2) The builder who erects very fine houses will make a large profit. (restrictive)
The first expression refers to an individual builder (and it implies that we know, or know of, the builder, which is the referent). It says that he builds "very fine" houses, and that he will make a large profit. It conveys these meanings by deploying a non-restrictive relative clause and three short intonation curves, usually marked-off by commas. The second expression refers not to a single builder but to a certain category, also called a set, of builders who meet a particular qualification, or distinguishing property: the one explained by the restrictive relative clause. 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. Commas are, however, often used erroneously, probably because the rule is taught based on logic and most people are not aware that they can trust their ear in deciding whether to use a comma. (English uses commas in some other cases based on grammar, not prosody.)
Thus, in speaking or writing English prose, a restrictive rather than non-restrictive meaning (or vice versa) requires the correct syntax by choosing the appropriate relative clause (i.e., restrictive or non-restrictive) and the appropriate intonation and punctuation.
To determine whether a relative clause is restrictive or non-restrictive a simple test can be applied. If the basic meaning of the sentence (the thought) is not changed by removing the relative clause, the relative clause is not essential to the basic thought and is non-restrictive. Alternatively, if the essential meaning of the thought is disturbed, the relative clause is restrictive.