For some people, it may be difficult to distinguish between the use of “who” and “whom“. So let’s see the differences between situations when you need to say “who” in English and as well as some usage tips.

Distinction

• “Who” is an interrogative pronoun and is used to ask a question. For example: Who is that?
  “Who” is also used in sentences where it replaces a subject. For example: Anyone who supports him should come with me.
• “Whom” is also an interrogative pronoun and also refers to a person, but, rather than replacing the subject in the sentence (the one who does an action), it is used instead of the person who suffers the action. For example:
  
  Whom is this song about? (Who is this song about?);
  Whom did they elect for president? (Who did they elect for president?)
  This is the high school teacher whom I just quarreled with. (This is the high school teacher that I just quarreled with.)
  They chose the person whom interviewed last. (They hired the man they interviewed last.)

It is important to remember to always use “whom” after a preposition (or when the verb to which it relates requires preposition, since when it is an indirect object, we must use “whom”):

Mary is the girl with whom I am going to spend the rest of my life with. 
(Mary is the girl I’m going to be with for the rest of my life).

It is worth noting that “whom” is considered to be formal. So, unless it appears directly after a preposition, marked even as an object rather than subject, in informal conversations, it ends up being replaced by “who.” Examples:

Whom are you going to fight with next? (Who are you going to fight with next?)

To whom should I complain to about the DVD? (With whom should I speak about the DVD?)