Yaesh's trick is correct - it is a way of telling whether you are referring to a subject or an object. Whom is only used for the grammatical object. However - the use of 'whom' is becoming very uncommon. You will almost never hear it in normal speech. Therefore - the simplest answer is to never use whom, although https://en.oxforddictionaries.com/usage/who-or-whom suggest there are some 'obligatory' cases.

"Whom" is just a different version of the word "who." They can be used interchangeably. Although, "whom" sounds more sophisticated and is generally used when speaking formally. "Who did you watch the movie with?" "Whom is it you are looking for?"

Hi there Jini There is a simple trick to do when in doubt that I teach in my lessons that I will share with you here. You would use "who" if you can answer the question with "he" or "she" Example: Who would like to go on vacation? She would like to go on vacation You would then use "whom" if you can answer the question with "him" or "her" Example: Whom do you believe? I believe him.

"Whom" is, formally, the object case (or "objective", or "accusative") of "who", though in much informal and spoken English its use is dying out https://en.oxforddictionaries.com/usage/who-or-whom

Who is in the main case. Whom is used when "who" is in any other case. "Who is doing this?" "Whom is it on" "To whom is it sent?" "Whom was it taken by?" "Whom was it written by?"