The document discusses the use of relative pronouns like who, which, that, and whom in English sentences. It provides examples of using these pronouns to refer to people as subjects or objects. For instance, it explains that "who" is used to refer to people as subjects, as in "She is the girl who wrote that article", while "whom" refers to people as objects, like in "We met Jane at the party, whom is the girl." The document also discusses using "which" and "that" to refer to things, animals, and objects.