This document discusses relative pronouns and relative clauses in English. It defines the different relative pronouns used for people, animals, things, time and place. It distinguishes between defining and non-defining relative clauses, noting that defining clauses do not use commas and the relative pronoun can sometimes be omitted, while non-defining clauses use commas. Examples are provided to illustrate defining and non-defining relative clauses. The document also discusses ambiguity that can arise and the placement of prepositions in formal and informal relative clauses.