Who or Whom?

The subject is who or what the sentence is about.
 The trip to the office usually takes about an hour.
The predicate is what the subject is or does.
The trip to the office usually takes about an hour.

“Who” is the subject case and “whom” is the object case.

Examples:
Who is going with him?[Who is the subject.]

She is going with whom?
[Whom is the object of the preposition with.]

To whom it may concern.
[Whom is the object of the preposition to.]

Questions – Inverted Word Order

Sometimes, questions use inverted word order (in which the verb frequently comes before the subject). To determine the subject and predicate, it may be necessary to reorder the words so that the subject comes first.

Examples: To whom should I address this email?
I should address this email to whom.

Who shall I say is calling?
I shall say who is calling.

Who do you think will win the award?
You do think who will win the award?

NOTE:

Substitute “he” or “she” for who in the sentence to hear if it sounds correct: Who ordered more supplies for the mailroom? He ordered more supplies for the mailroom.

Substitute “him” or “her” for whom in the sentence to hear if it sounds correct: I should address this email to whom? I should address this email to her.