WHO - WHOM

There is often confusion about the use of who and whom. 
Who and whom are pronouns. 
Who is a subject pronoun, in the same way as 'he/she/they'. 
Whom is an object pronoun, in the same way as 'him/her/them'. 

In the sentence "John loves Julie." :

• John is the subject of the verb 'love'
• Julie is the object of John's affection.

Simple rule : If you can substitute 'he/they', use 'who'. 
 If you can substitute 'him/them', use 'whom'.

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Subject	Verb	Object
Tom	loves	Julie
Who	loves	Julie?
Tom	loves	whom?

Subject

• Who
  
  We use 'who' when it is the subject of a verb, that is, when it refers to the person who
  takes an action.
  
  • Julie played tennis. Julie is the subject of the verb 'to play'.
  • To find out the name of the player, we ask a question using 'who'.
    Who played tennis? Julie played tennis.
  • Who can also be used as the subject of a non-identifying clause:
    • There's Mr. Jones who bought the house next door.

Object

• Whom
  We can use 'whom' as the object of a verb, but it is very formal and not often used in spoken English.
  
  • Formal English :     Whom did you see?
  • Everyday English :  Who did you see?
• In formal English, whom is used directly after a preposition:
  • With whom did you play?
  • In informal conversational English, it is more usual to ask :
    • Who did you play with?
• Whom is always used when it is preceded by quantifiers such as all of, both of, few of,many of, several of, etc. For example:
  • He addressed the spectators, most of whom remained seated.