A relative pronoun is used to connect a clause or phrase to a noun or pronoun. The clause modifies, or describes, the noun.
The most common relative pronouns are who, whom, whose, which, and that. Sometimes when and where can be used as relative pronouns as well.
Relative pronouns are placed directly after the noun or pronoun they modify. For example:
In each example above, the subject of the sentence is described by a relative clause (italicized). As these clauses describe a noun or a pronoun, they are also known as adjective clauses, because they act like adjectives in the sentence. Each clause is introduced by a relative pronoun (in bold). Relative pronouns connect the description to the rest of the sentence in an orderly way.
Occasionally, the relative adverbs "when" and "where" are also used as relative pronouns. For example:
In these cases, "when" and "where" introduce clauses that describe a noun the refers to a time or place, making them work as relative pronouns in these sentences.
When relative pronouns are used to add descriptive information, that information is either defining or non-defining. A defining clause - also known as a restrictive clause - gives essential information about the noun in question. It is so important that it cannot be cut out of the sentence and still convey the intended meaning. For example:
In both cases, the italicized clauses contain critical information. You can tell because if you cut out the clause, the sentence's meaning is fundamentally different. For example, saying "I don't like people" is very different from saying "I don't like people who interrupt me."
Note that defining clauses require no additional punctuation.
On the other hand, non-defining clauses add information that's nice to have but isn't essential to the sentence's overall meaning. They could be deleted and the sentence would convey basically the same information. For example:
In both cases, you could cut out the non-defining clause and still understand the point of the sentence. The important part is that the paint is worth a million dollars; the fact that it is adored is merely nice to know.
Note that non-defining clauses are set apart from the main sentence by commas, which help to indicate its less important status in the sentence.
One of the most common mistakes in writing is to use the wrong relative pronoun, particularly when it comes to mixing up "who" and "that." "Who" is always used to set up a relative clause that describes a person, while "that" is used to describe an object or another non-human being. For example:
Another common error is to mix up that and which. When describing objects and non-human beings, "that" is used to introduce a defining relative clause, while "which" is used to introduce a non-defining clause. For example:
The relative pronoun "which" is used for non-essential information set off by commas; "that" is used for essential information and requires no additional punctuation.
When it comes to people, however, you don't have to worry about confusing "which" or "that." You always use "who:"
Knowing how relative pronouns work in a sentence will help you add important descriptive information in the form of relative clauses. Once you understand how they work, you'll be able to decide whether your information is defining or non-defining and choose the appropriate relative pronouns and punctuation to lead your readers to a deeper understanding of your meaning.