Relative clauses provide additional information about a noun and are introduced by relative pronouns. Defining relative clauses identify the noun and use no commas: "The student who won the prize is here." Non-defining relative clauses add non-essential information and are set off by commas: "My sister, who lives in Paris, is visiting." Removing a non-defining clause does not change the main meaning of the sentence.
The choice of relative pronoun depends on its function and the type of clause. "Who" refers to people in subject position ("The man who called"), while "whom" refers to people in object position ("The man whom I met"). "Which" is used for things, often in non-defining clauses. "That" can substitute for "who" or "which" in defining clauses, especially with superlatives or after words like "all" or "everything." "Whose" shows possession ("The woman whose car was stolen"). A common error is confusing "whose" (possessive) with "who's" (contraction of "who is" or "who has"). "Whoever," "whatever," "whichever," and "whomever" are compound relative pronouns referring to unspecified people or things ("Whoever arrives first should wait").
Reduced relative clauses and participle clauses make writing more concise by omitting the relative pronoun and sometimes auxiliary verbs. A present participle (-ing) replaces an active relative clause: "The man standing by the door is my father" reduces "The man who is standing by the door." Past participles replace passive or completed actions: "Finished with the work, she left." Participle clauses also condense adverbial clauses: "Walking down the street, I saw a friend" replaces "While I was walking down the street." Cleft sentences emphasize specific information using "it" or "what" with "be," as in "It was John who called" or "What I need is coffee."