Whom is a form of the word “who” that you use when referring to someone receiving an action rather than doing it. Think of it this way: you use “who” when someone does something (like “Who threw the ball?”), but you use “whom” when something is done to someone (like “To whom should I throw the ball?”).

Here's a simple test: if you could answer the question with “him” or “her,” use whom. If you'd answer with “he” or “she,” use who. For example, “Whom did you call?” works because you'd answer “I called him.” But “Who called you?” works because you'd answer “He called me.”

In everyday conversation, many people skip whom and just use who for everything, which is fine for casual speech. But in formal writing, using whom correctly shows precision and care with language. You'll see it most often in questions like “For whom is this gift?” or after words like to, for, and with, as in “the teacher with whom I spoke” or “the friend to whom I wrote a letter.”

Learning when to use whom is like learning the difference between I and me: both are forms of the same idea, but they work in different places in a sentence.