I would imagine that "Who" is nominative case, used as subject, where as "whom" is accusative case, used as object.
There was an old television game show, "Who do you trust?" People often joked that in should more properly be "Whom do you trust."
There was an old cigarette ad, "Winston tastes good LIKE a cigarette should."
Supposedly, it should more properly read "AS a cigarette should."
People usually say "good" when what they mean is "well", since good is adjectival while "well" is adverbial.
Another fun topic, somewhat archaic, is the distinction between "will" and "shall".
"Shall" denotes that which is routine, and not the result of determined intentionality to act out of the ordinary, whereas "will" denotes the concerted effort of ones will to step outside what is routine.
"I shall be in the office until six pm" means that your normal hours are from 9am until 6pm. If you say, "I will remain until 7pm to finish my project" then you are exerting your will to deviate from your normal scheduled hours.
This is my understanding of "will" and "shall", at any rate, in my own words. If I am mistaken, then my sincere apologies. There are various books on style and grammar which explore these arcane and archaic rules and usages.
I did see a large and expensive book entitled something like "The Chicago Manual of Style." I would love to have something like that, and go through it meticulously, but I have neither the shelf space, nor the budget, nor the youth and energy.
And there likely to go off to.
