Relative clauses typically act as adjectives and follow the word or phrase that they modify. (In the prior sentence, "that they modify" is a relative [adjective] clause.) Relative clauses begin with a relative pronoun (who, whom, which, that) and contain both a subject (typically the relative pronoun) and a verb (though not the main subject and verb of the sentence).
You may find it useful to think of a relative clause as a sentence that has been embedded in another sentence. Information contained in this embedded sentence becomes subordinate to the information contained in the main clause of the sentence.
In the following sentences the relative clause is in bold.
Biff recently won a Nobel Prize for music.
Biff has written a number of novels and short stories
Biff, who has written a number of novels and short stories, recently won a Nobel Prize for music. Biff wrote a song called Computer Company."
The song is about a cowboy.
The cowboy trades his horse for a computer.
Biff wrote a song called "Computer Company," which is about a cowboy who trades his horse for a computer. Cowboy Bob owns a ranch.
The ranch is the largest in Minnesota.
Cowboy Bob owns a ranch that is the largest in Minnesota. Cowboy Bob asked Biff for help.
Cowboy Bob's cattle had been rustled.
Cowboy Bob, whose cattle had been rustled, asked Biff for help. Biff suspected Swen Swenson.
He had fought him many times before.
Biff suspected Swen Swenson, whom he had fought many times before.
Your choice of relative pronoun will depend partly on meaning and partly on grammar. Who and whom refer to people. Who is for subjects and whom for objects of clauses. Whose shows possession. That is typically used in restrictive clauses (those clauses necessary for understanding the meaning of the sentence); which is typically used for nonrestrictive (nonessential) clauses (those clauses typically set off by commas). When clauses follow a proper noun (a name), they are nonrestrictive clauses and are set off by commas.
Cut and paste the following sentences to a document open in your wordprocessing program (or open Relative.doc); then, combine the sentences by turning one (or more) of the sentences into a relative clause. Use the first sentence in each group as the base clause.