First, let’s define subject pronouns and object pronouns.
Subject pronouns are the actors of the sentence: I, you, he, she, we and they are common subject pronouns.
Object pronouns are the receivers of the action. Me, you, him, her, us and them are common object pronouns.
"Who” is a also a subject pronoun, so use “who” in places where you would say “he” or “she”:
“Who wants to know?” “Who is it?” (which explains why “it is he” is the correct usage, not “it is him”) “Who is your favorite writer?"
Conversely, "whom” is an object pronoun. Use “whom” in places where you would say “him” or “her”:
“To whom are you speaking?” (“I am speaking to her.”) “To whom was this letter sent?” (“It was sent to him.”)
However, “who” can also serve as a modifier (a word used to describe someone further), which can create some confusion among those who understand the correct usage of “whom” in all other instances.
The rule is easy: “whom” should never be used as an adjectival modifier, no matter whether it refers to the subject or the object of a sentence. In other words, if the word is used to describe someone already mentioned, then you should always say "who", not "whom".
“I am looking for a tutor whom understands every in and out of the GMAT verbal” = incorrect “I am looking for a tutor who understands every in and out of the GMAT verbal” = correct
Why? Because the object of these sentences is the word “tutor”, not the word “who.” In this case, “who” serves as more of an adjectival modifier to the main object.