Spoilers: We’ll be diving into who vs. whom in this one!
The first thing we should mention is that relative pronouns introduce relative clauses. A relative clause is a type of dependent clause (a clause that can’t stand by itself as a complete sentence). It adds extra information to a sentence. The five relative pronouns are who, whom, whose, which, and that.
Who vs. Whom
Who among us hasn’t struggled with this one? Who is a subject pronoun, which means it can be used as the subject of a sentence or clause. It can only be used to refer to people. For example, “Do you know who called earlier?” Here, the main clause is Do you know? The main verb know applies to the dependent clause, who called earlier. Who performs the action call, which makes it the subject of the dependent clause.
Whom also applies to people. It’s an object pronoun, which means it describes a person on the receiving end of an action. For example, “I don’t know for whom she called.” The main clause is I don’t know. The relative clause (for whom she called) explains what the speaker knows. She performs the action (calling), and whom receives it. This makes whom the object of the dependent clause.
A good way to decide whether you should use who or whom is the he vs. him rule. Both him and whom end with M. If you can substitute him into a sentence (or answer a question with it) you should use whom. If he works better, you should use who. For example, you can answer “Whom did you call earlier?” with “I called him,” so whom is the best choice here.
Whose is a possessive pronoun. It gives more information about a person by describing things that belong to them. Take this example: “This park is for people whose dogs need a place to play.” The park isn’t for all people. It’s only for those with dogs who need a place to play. So, whose dogs need a place to play is a dependent clause. Whose describes the kinds of people who can use the park.
You’ll tend to see whose in questions like “Whose phone is this?” and “Whose glass is that?”
Which vs. That
Which and that are relative pronouns that apply to objects. In general, you’ll see which in relative clauses that aren’t crucial to a sentence. Key clauses tend to use that.
For example, “The hotel, which had an indoor pool, was also pet-friendly.” The information about the pool isn’t crucial to the meaning of the sentence. It just gives more information about the hotel. Contrast that with “The hotel that had an indoor pool was more expensive than the one that didn’t.” This sentence points out one specific hotel and compares it to another. That had an indoor pool and that didn’t are relative clauses that are both crucial to the meaning of the sentence.