The words that connect clauses to nouns they modify
Relative pronouns introduce relative clauses, which provide additional information about a noun. The main relative pronouns are who, whom, whose, which, and that. They serve two key functions:
In informal US English, "who" often replaces "whom" ("The person who I met"). UK English more commonly retains "whom" in formal writing.
Required after prepositions in formal writing ("For whom was this intended?"). Often replaced by "who" in speech.
"A book the cover of which was torn." (more formal)
US English prefers "that" for restrictive clauses ("The house that we bought"). UK accepts both.
Never use commas with "that" clauses – they’re always restrictive.
When the pronoun is the subject of its clause ("The man who called you" – not "The man called you")