"Who" vs "whom" - when to use each

Who is the subject - the one doing something.
Whom is the object - the one something is done to.

Replace "who/whom" with "he" or "him." If "he" sounds right → use "who." If "him" sounds right → use "whom."

"Who/whom called?" → "He called" ✓ → Who called.
"Who/whom did you call?" → "You called him" ✓ → You called whom.

• "Who is coming?" ✓ (he is coming)
• "To whom should I address this?" ✓ (address it to him)
• "Whom did you speak to?" ✓ (you spoke to him)

"Whom" is gradually disappearing in spoken English. Many native speakers never use it in casual speech. In formal writing, legal documents, and professional emails, using "whom" correctly signals sophistication. In conversation, nobody will correct you for saying "who."