Use who when the word is doing the action. Use whom when the word is receiving the action or following a preposition.

So the basic rule is:

✅ Who called you?

Here, who is doing the action of calling.

✅ Whom did you call?

Here, whom receives the action.

✅ To whom am I speaking?

Purdue OWL notes that whom is the standard formal choice after a preposition.

✅ Who am I speaking to?

Merriam-Webster notes that in ordinary speech and writing, many speakers prefer who instead of whom, especially outside very formal contexts.

The most common mistake is assuming you must use whom whenever you want to sound educated. That usually backfires. Merriam-Webster and Purdue OWL both note that whom now has a formal feel and is much less common in everyday speech. In many normal conversations, who sounds more natural.

Another common mistake is mixing up who and whom in questions:

❌ Whom is coming to dinner?
✅ Who is coming to dinner?

If the word is the subject, who is the correct choice.

Use this memory trick:

If he or she sounds right, use who.
If him or her sounds right, use whom. Grammarly recommends this kind of pronoun-substitution test for choosing correctly.

A shorter version:

who does it
whom receives it