The document discusses the use of personal pronouns to replace nouns referring to objects and phenomena as subjects in sentences. It provides examples of replacing subjects like "ball", "watch", "rainbow", and "weather" with third person singular pronouns like "it", "he", "she", and "it" depending on the gender of the noun being replaced. The document also notes that third person singular and plural pronouns of all three genders can substitute for common noun subjects that refer to objects or phenomena.