I remember in the fourth grade, my English teacher said "I don't think there's a difference" and this woman said whom all the time.

But she was FUCKING WRONG

I SHALL EXPLAIN THE DIFFERENCE!

Who is used when referring to someone that you don't know the gender of (they, it)

Whom is used when referring to a specific gender (he, she)

For example;

"Who are they?"

"Whom is he?"

Now you can brag about your sense of information to others by using correct grammar.