The difference between “who” and “whom” lies in their grammatical function within a sentence:

1. “Who” is a subject pronoun, meaning it is used to refer to the subject of a sentence or clause.
   • Example: Who is coming to the party? (Here, “who” is the subject of the verb “is coming.”)
2. “Whom” is a object pronoun, meaning it is used to refer to the object of a verb or preposition.
   • Example: Whom did you invite to the party? (Here, “whom” is the object of the verb “invite.”)
   • Example: To whom should I address the letter? (Here, “whom” is the object of the preposition “to.”)

Quick Rule:

• Use “who” when referring to the subject.
• Use “whom” when referring to the object.

In informal speech, “who” is often used in place of “whom”, especially in conversational contexts. However, in formal writing, following this rule helps maintain grammatical correctness.