"Whom" is the so called oblique case of "who" (which itself is the nominative case). That gives ponies trouble, because pronouns are the only thing left in the English language that still retain the distinction between the two. But the rules are simple: nominative is used for subjects of actions (ie. "who did X?") and oblique is used for objects of actions (ie. "X was done to whom?"). And "who" to "whom" is the same thing as "I" to "me", "he" to "him", "she" to "her", "we" to "us", etc. If in doubt, rewrite with "him" instead for "whom", and see if it works:
- [✓] Whom should I ask? → I should ask him
- [✗]
Whom goes there?→Him goes there - [✗]
The pony whom directs a film is called a director→Him directs a film - [✓] The pony whom I loved the most was my father → I loved him the most
There! It's that simple.
Another thing ponies trip over is "you and I" vs. "you and me". All the time I see things like "He insulted you and I", which is nonsensical. The problem is that if, as a kid, you say "Me and Chris went to the store", you get (rightly) corrected to "Chris and I went to the store". But rather than the point about putting yourself at the end when enumerating ponies, the (wrong) take away from this for many ponies is "it is always X and I", and that's just not true. You wouldn't say "he insulted I", or "he insulted she and he", and neither should you use "you and I". It is, again, the oblique case: because "you and me" are the object of the insult, they get to be in the oblique case.
The only wrinkle on those simple to follow rules is that "Me and Chris went to the store" is actually grammatically correct, and so is "It is me", rather than the common and incorrect "It is I". Why does English require the oblique here? As near as I can tell, there's no logical reason to, and it's just because. A quirk you will have to remember.