Who vs Whom
If you want to know a REALLY gay way to know when to use the right form, all you have to do is watch for the "m" in the answer (the hypothetical answer, which has to always be masculine- see below).
If you are wondering "To Who it may concern" or "To Whom it may concern" (as an example), the hypothetical answer is "it concerns HIM". Since the masculine answer in this is HIM and not HE, you add the "m" (Whom).
If you are wondering "Who has their hand on my ass" or "Whom has their hand on my ass", the answer would be "HE has his hand on my ass" (and NOT "HIM has his hand on my ass"- unless you are Tristan). Note that HE has no "m".
It's really a gay method, but it is foolproof, fools. All you have to do is a quick mental calculation with the answer. HIM=Whom and HE-Who.