Who vs. Whom
Forget what your teacher said. Seriously, it’s super easy, and I’ve noticed that people either make it ridiculously hard or disregard it entirely and use only “who”.
Here is an incorrect sentence: She whom decided this was going to be rich. Here is a correct sentence: She who decided this was going to be rich.
The real sentence sounds right because it uses “who” before a verb. This is why “whom” does not work.
Here is an incorrect sentence: He who we loved was no more. Here is a correct sentence: He whom we loved was no more.
What’s the difference? The correct one uses “whom”, but why? Because it’s followed by a subject (pronoun).
If the word after the who/whom (or whoever/whomever) is a verb, use “who/whoever”. If the word after the who/whom (or whoever/whomever) is a subject, use “whom/whomever”.
Once you notice it, it’s hard to forget, but maybe that’s just me.