Look, honestly, if you want to sound normal, just use who for everything. That’s so okay that if you search whom on Wikipedia it just goes straight to the article on who. However, if you want to know enough to be able to judge people on when they use it wrong, here’s the deal. Who is the subjective pronoun, like I or he - it does the verb. Whom is the objective pronoun, like me or her - it has verbs (or prepositions) done to it.
Here are some correct usages:
Who touched my vintage Pokemon cards? I was going to do battles with those tomorrow!
She was the one whom he insulted when he said he wouldn’t date any women who preferred Samsung televisions over Sony ones. Yeah, I think that’s weird too.
Here are some incorrect usages:
Whom said that my farts smell more than average? How does she even know that?
Flo-rida, whom many think has the best Florida-based rapper name, is inferior in both name and music quality to T-Pain.
Honestly, I don’t know anyone has ever, ever said my second incorrect example. Anyway, now you know the difference. However, I want to reemphasize that you can live a very full life not every saying whom. Also, Shakespeare has used it “wrongly”, so whatever. Here’s more stuff from Wikipedia:
“According to the OED (2nd edition, 1989), whom is "no longer current in natural colloquial speech". Lasnik and Sobin argue that surviving occurrences of whom are not part of ordinary English grammar, but the result of extra-grammatical rules for producing "prestige" forms.”
But we are all about prestige forms. Well, I am. Also, now that you know, you can definitely silently (or vocally) judge the grammatically inferior. Also, if you hear someone say whom out loud, you can call them a prescriptivist as a weird insult, which my sister did to me once. If they are confused, you can revel in your superior vocabulary. Just don’t call yourself a Grammar Nazi. Generally, don’t call yourself any kind of Nazi. That’s a good life policy.