Before "whom" becomes all but forgotten, I thought I would take the time to detail this rather complicated usage rule in hopes of simplifying the process for others. Frankly, it's a usage rule that probably shouldn't exist in English anymore, as it really serves no practical purpose. The fact that one must sometimes make a conscious effort to distinguish subject and object in order to understand when "whom" is called for also complicates matters further. Professional writers have sometimes encouraged people to either never worry about this rule, or reword themselves to avoid necessitating its appearance altogether. On the other hand, the usage of "whom" can aid one in having a more formal speaking manner.

First, it's important to understand why "whom" is used in the first place. Usage of "whom" indicates the object of a clause to your listener(s). On the other hand, the rule is rather irrelevant, as a listener can distinguish subject from object on their own. This really just leaves a superfluous burden on you to make this distinction for others. In sentences where a subject is only implied, I suppose it makes more sense to indicate that someone else is an object; but it's still understood.

Secondly, it's important to understand the functions of a subject and an object in a sentence. Obviously, the subject is the subject - the very thing being discussed. If I say, "I'm going to the market," I am the subject - the person going to the market. The market is the object. It gets more confusing when you include other people in a sentence, or have a sentence that includes many different objects. Just remember that anytime you're speaking about yourself, you're the subject. If you don't appear in the sentence, anyone else you speak about is likely the subject. It just depends on who or what (subject) is doing what to whom or what (object) ... If that makes any sense.

One easier way to look at this is to consider whether or not a preposition is appearing in your sentence. (of, for, to, from, over, toward, between, etc.) These show some relation to an object. If I say, "I spoke with him today," "to" is the preposition that indicates my relation (the subject) to "him." (the object) Now, if the sentence were something like this, "With whom where you speaking today?" we notice that "whom" is used, because it describes the object. (the thing modified by a preposition) "You" is the subject, in this case. It's tricky at first, but you get the hang of it. This is probably the easiest way to remember whether or not to use "whom," as opposed to consciously distinguishing between subject and object. As long as you understand good and well what a preposition is, you're pretty much set.

On a final note, I wanted to elaborate on relative pronouns, as well. These are less direct usages of who, that, which, etc. It's important to use the right pronouns, as some are reserved for sentient beings only. If you mix and match them, it can sound like you're implying a person is a physical object rather than a person. Example:

"The person who went to the store." Proper. "Who" indicates the person is a human being.

"The person that went to the store." Improper. "That" implies the person is a physical object. The only time you use "that" is in a sentence like, "The book that fell from the shelf." The book is a physical object, so you wouldn't refer to it any other way.

In some way, using "who" as a relative pronoun is almost like a mandatory honorific. If you imply that a person is anything other than a person, it can be misconstrued. (sort of like referring to a person as "it," instead of using a gender pronoun) This might be a rare occurrence, but still...

OBTW, in German you not only have a subjective (called nominative) case, an objective (called accusative) case, but ALSO you get to have fun with the DATIVE case and the GENETIVE case. then it gets more interesting with accusative and dative prepositions! did i mention some prepositions can be either dative or accusative depending upon whether or not the verb is considered a motion verb? also, all indirect objects are considered of the genetive case and all direct objects of the accusative. the genetive case is associated with possession. lastly, some direct objects may be dative rather than accusative if the verb is considered a "dative requiring verb". crazy right? i love grammar.

Hurray for German!

Note: This was in response to nemo's post, Zera just beat me to posting.

I is taking German 4 next year. Good luck with that teacher though.

I'm taking Latin (Latin 4 next year), which partially explains my grammatical knowledge.
It's not too bad once you get used to it, though it is still a pain. (Non-living things have pretty unpredictable gender! yay! )

nonliving genders are the worst. i can never remember whether my desk is a dude or my lamp is a girl. for the record, my desk IS a guy and yes, lamps are girls. at least in german. there are still some traces of this in english (ships being called "she") though.