The document discusses the use of relative pronouns who, whom, which, and that. It explains that these pronouns are used to combine two sentences into one by referring back to a noun. It provides examples of using who for people, which for animals, things, and sometimes groups, and that which is often substituted for whom today. The document also discusses restrictive and nonrestrictive clauses and using commas appropriately. It includes exercises for readers to practice identifying and using relative pronouns correctly in sentences.