From The Office: Season 4, Episode 4: "Money"

When referring to the object of a sentence use the word Whom. Only use the word Who when referring to the Subject of the sentence:
Ex: Subject
Who is buying the movie tickets?
The person is the subject of the sentence.
Ex: Object
The movie tickets were purchased by whom?
In this case, the tickets are the subject of the sentence.

It's a subtle difference. In fact, so many people are confused by this one that they almost always structure sentences to avoid using the word Whom. As language evolves it looks like this is one word that will eventually go away.

One trick to remember it is that a subject can never be in a prepositional phrase, like in the 2nd example, by whom. See this link for more info about finding Subjects/Verbs/Objects in sentences.

Last modified: Wednesday, February 29, 2012, 2:40 PM