Who and Whom

Use "who" when referring to, or asking about, the subject of a sentence, and "whom" when referring to, or asking about, the object.

The subject is the person or thing performing an action, for example:

He went to the library.

He is the one going to the library, thus "he" is the subject.

The object is the person or thing receiving the action, for example:

He went to the library.

The library is where "he" is going; it is receiving the action from the subject, and is thus the object of the sentence.

Here's a good mnemonic from grammar girl:

If I say, "I love you," you are the object of my affection, and you is also the object of the sentence (because I am loving you, making me the subject and you the object).

Now some examples of when to use "who" and "whom":

Whom did you go to the beach with? (Asks about the object, not the subject, "you")

Who went to the beach? (Asks who was performing the action--going to the beach--thus asks about the subject)

The one whom he loved was beautiful in every way. (Refers to the object that "he," the subject, loved)

The one who loved him was beautiful in every way. (Refers to the subject that loved "him," the object)

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