Who VS Whom

I was reading the difference between "who" and "whom". This is what I found out:

"Who" and "whom" are both interrogative pronouns. However, the difference between these interrogative pronouns is that the first one (who) is used in place of the subject of a question, while the second one (whom) is used in place of the object of a question.

Here below are some examples:

-Who as a subject: 

Who are you?

Who is going?

-Whom as an object:

Whom did they tell?

Whom is the story about?

P.s

-Whom is always the correct choice after a preposition.

E.g: Leonela is the girl with whom I have classes in Santo Domingo.