This document discusses relative clauses and relative pronouns. It defines relative clauses as subordinate clauses that provide information about a noun, called the antecedent. Relative clauses begin with a relative pronoun like which, who, when, where, or whose. There are two types of relative clauses - defining clauses that are essential to the meaning and non-defining clauses that provide extra information. The document explains the different relative pronouns and when they can be substituted or omitted from relative clauses.