Who Knows? Or whom knows?... Who cares

So, this week in my News Writing Grammar class, we were instructed to decipher the difference between who and whom. As Professor Young is telling us to do this, I'm thinking, does anyone in the World even know the difference? But, I guess someone does because when I asked Google, a man called the "Grammar Monster"gave me the answer... Apparently when you use who it must be when you are referring to the subject of a verb. Sounds confusing. Whom, on the other hand, is NEVER used as the subject of a verb, but after a preposition! You know what a preposition is right? There is a whole slew of them (ex: by, with, to, etc.). Golly I didn't think this could have gotten any more confusing. However, I learned that if you are unsure of what to use, resort to who. Who is more commonly used and most likely the proper tense. It's hard to think before speaking, but if you're unsure of what to use [who vs. whom] you gotta use your brain! So I guess you can add one more person to your list of people that know the difference between who and whom... and then you can add yourself!! Whoever you may be...