Refers to people. Use "who" when the relative clause describes a person. Example: "The author [who] wrote the bestseller is coming to our book club."
Refers to things and animals. Use "which" when the relative clause describes a thing or an animal. Example: "The laptop, [which] I bought last week, is very fast."
Can refer to both people and things. Use "that" for restrictive relative clauses, which are essential to the sentence's meaning. Example: "The book [that] I recommended is a classic."
Mastering relative clauses and pronouns will enhance your writing and make your sentences more precise.