Many times, I've been asked the difference between "who" and "whom". I myself know the difference, but it is hard to explain to others. What is the easiest way to explain it to those with a basic understanding of English? A mnemonic (if possible) would help.
In modern informal usage
Traditionally, who and whom are distinguished: who is used as a subject, whom as an object. But the distinction produces a number of inconsistencies, and it has confused even such native speakers as William Shakespeare, the translators of the King James Bible, and Daniel Defoe.1
Fortunately for the English learner, in modern informal English, you don't need to distinguish them. Who (as well as derived forms like whoever) can be used in all positions.
For instance, who can be used as both a subject and an object (as an interrogative):
Whosubject of "[i]s"'s there?
They sacked whoobject of "sacked"?
Whoobject of "to" will the task be assigned to?
and similarly when it heads a relative clause:
Give it to anyone who asks for it.
Give it to the employee who it's assigned to.
"Whom" will generally still be used when it immediately follows the preposition it is the object of. But such constructions are uncommon in informal English, since pied-piping of prepositions is used only in formal registers.
"Whomever" is essentially unused, with "whoever" used even immediately after a preposition:
Give it to whoeverobject of "to", subject of "asks" asks for it.
Give it to whoeverobject of "to", object of "picks" the computer picks.
1 That is, the distinction was never widely observed in detail.
A long time ago,
in a galaxy far, far away, English used to have noun cases. Generally, cases where the rules by which a noun changes.
When a noun was a primary actor in a sentence, it was used in a nominative case.
However, if a noun was an object of some action, other cases were used.
My brother is a doctor - here, "brother" is a primary subject, so nominative case was used;
I give an apple to my brother - here, "brother" is not a primary actor, so the noun used another case. Usually, dative;
I see my brother - here, "brother" is a direct object of an action, so the noun was in objective case.
There are many resemblances of former use of noun cases, and the most prominent example is pronouns:
Nominative "I", but objective "me";
Nominative "who", but objective "whom"
Hence, the answer to your question:
Who is used when you are talking about the subject (actor):
Who broke the window?
Whom is used when we are talking about an object of a certain action:
With whom are you going to a school?
Who is Subject Oriented Interrogative pronoun and Whom is Object Oriented Interrogative Pronoun.
Usage of Who:-
1) It can be used in place of subject to interrogate.
Ex- i) Who are you?
ii) Who taught you to talk like that?
2) It can also be used as a subject of a clause.
Ex- i) This is the person who was searching for you.
ii) Anybody who would like to climb the hill may come forward.
Usage of Whom:-
1) It can be used in place of object to interrogate.
Ex- i) Whom do you want to leave with?
ii) Whom is this present for?
2) It can also be used as a object of a clause.
Ex- i) This is the person whom I told you about.
3) Whom is always the correct choice after a preposition.
Ex- Katrina is the girl with whom I am going to date.
I taught this to my students in a very simple manner just last week.
Who broke the vase? Whom did mother call?
In the first sentence, the work is done by the unknown person 'who'. It is in active voice. Hence, we use 'who'.
In the second sentence, 'mother' does the work. The verb refers to the object. It is in passive voice. Hence, we use 'whom'.
In other words, if the unknown entity does the verb, use 'who'. If the verb is done on the unknown entity, use 'whom'.