RELATIVE PRONOUNS They are called so because they serve to introduce relative clauses, i. e. a variety of attributive clauses, which are always correlated with some antecedent in the principal clause. The relatives are used as adjective-pronouns.
Relative pronouns, like interrogative and conjunctive ones, discriminate, animate and inanimate objects: who/whom is used for persons, which — for things; whose and that are used for both persons and things.
Relative clauses are of two kinds: (1) restrictive and (2) non-restrictive. The use of relative pronouns has some peculiarities in each of these two kinds of clauses.
1. Relative Pronouns in Restrictive Clauses Restrictive clauses characterize the preceding noun in such a way as to distinguish it from other nouns of the same class. A clause of this kind is essential to the clear understanding of the noun. It cannot be omitted.
The pronouns who/whom and that are interchangeable when used for persons in such clauses.
The man who robbed you has been arrested.
But that (not who) is necessary after superlatives and certain pronouns (all, everything, everyone, no, none, not any, nothing, nobody, much and little).
He was one of the greatest writers that ever lived. I have nothing that would fit you. Much that I have read has been nonsense. "Are there any bookshops in this street?" "None that I know of."
The objective whom is considered very formal and seldom used in spoken English: who or that (that being more usual than who) are used instead, and it is still more common to omit the relative pronoun altogether.
The man whom I saw was called Smith. The man who I saw is called Smith. = The man that I saw...= The man I saw...
With a preposition, whom or that may be used.
The man to whom I spoke was our dean. (This is formal but quite often used.)
In informal speech, however, it is more common to move the preposition to the end of the clause and either change whom to that, or omit the relative pronoun altogether.
The man to whom I spoke... (formal but possible even in spoken English) = The man that, I spoke to... (used in spoken English) = The man I spoke to... (much used in spoken English)
The possessive whose stands for persons and, occasionally, for things.
The people whose houses were damaged will be compensated. Living in a house whose walls were made of glass would be horrible. (The latter is, however, often replaced by a phrase, i. e. ...with glass walls....)
The pronouns which or that are interchangeable when used for things: which is more formal and is chiefly used in written English, that is more common in speech.
The book which caused such a sensation was sold out within a week. The dog that won the race is called Rex.
When these pronouns are used as objects in restrictive clauses they can be omitted.
The book which I bought is very interesting. = The book that I bought... = The book I bought...
The preposition is placed immediately before which in formal English.
The address to which I wrote was wrong.
In spoken English the preposition is moved to the end of the clause and which is either changed to that or omitted.
The address that I wrote to was wrong. == The address I wrote to.;.