I am often asked to explain the difference between who and whom. This is a tricky one for all of us, even grammar junkies.
Many people think whom is simply a formal and antiquated way of saying who. Slang and regional dialects are edging in on basic rules once set in place to streamline language. Who vs. whom is increasingly easy to ignore as society is becoming lax and devolving language.
Who and whom are relative pronouns, each specifically used as the principles of grammar dictate. One is no more or less formal than the other. They are interchangeable, yet swapping one for the other changes the meaning of a clause.
RULE: Use who when you are referring to the subject, the person doing the doing.
RULE: Use whom when you are referring to the object, the person the doing is being done to.
There is an easy-peasy way to figure out the who vs. whom mystery. “Remember the letter M.”
Who and whom are words used in questions. Answering the question will give a hint as to which relative pronoun should be used.
Answer: He is taking her to the movie.
Question: Who is taking her to the movie?
No: You would not say, “Him is taking her to the movie.”
Following the Rule: He ≠ M ≠ whom = who.
Answer: She is going to the movie with him.
Question: With whom is she going to the movie?
No: You would not say, “She is going to the movie with he.”
Following the Rule: Him = M = whom ≠ who.