Would you say, "Pass I the salt"? Let's hope not. You would say, "Pass me the salt." And of course, you should add "please." The sentence is a shorter version of "Pass the salt to me." Me is the object of the preposition to and therefore you use the objective form me instead of I, the subjective form. Saying "Pass I the salt" would be the same thing as saying "Pass the salt to I." For the same reason, you would say, "I passed the salt to him," not "I passed the salt to he" or "I passed he the salt." In these cases, he is the object of the preposition to, just as me is in the first example.
So far, so good. Few people have trouble with any of that. However, they get confused when two words are simultaneously the object of the same preposition. For example, if you say, "Pass the salt to John and Sally," both John and Sally are objects of the preposition to. If you think about it, that makes sense. You're saying that the salt should be passed to both of them - "Pass the salt to John and [to] Sally" - so it's the same situation as if you had only named one of them. You would say, "Pass the salt to him," or "Pass the salt to her." Therefore, you could and should say, "Pass the salt to him and her."
For exactly the same reason, you would say, "Pass the salt to John and me." Never say, "Pass the salt to John and I." Or "Give the report to John and I." Just as above, John and me are both, together, simultaneously, in sweet harmony, objects of the preposition to. To hark back a couple of paragraphs, you wouldn't say, "Give the report to I." That's why you shouldn't say, "Give the report to John and I."
I, he, and she are the subjective forms. They are used as the subject of a sentence. "I did this." "He said that." "She went there." Me, him, and her are the objective forms. They are used when the word receives the action of a verb or preposition. "I stopped him." "He gave the book to her and me." "She asked him a question."
Between is also a preposition. That means that the words it acts on have to be in the objective case. That's why it's incorrect to say, "Between you and I." The correct form is "Between you and me." It's the same logic as the stuff above about to.
This kind of mistake - using I instead of me - is sometimes called "hypercorrection". The miscreant thinks he's being more elegant or more grammatically correct if he avoids me. He doesn't realize that those in the know are pointing at him and laughing and jeering. Don't assume that I is always somehow more correct than me. It ain't.
A related error, curiously enough, is the opposite of the above. People very often use the objective form of who or whoever when they should be using the subjective form. For example: "Give the book to whomever answers the door." In this case, I suspect the speaker knows about prepositions and their objects but doesn't understand what the object actually is in this sentence. At first glance, it may seem that whomever is the object and therefore should have the objective form. But in fact, the object is the whole clause whoever answers the door, and whoever is the subject of that clause. So the sentence should be "Give the book to whoever answers the door." That is, give the book to the person who answers the door.
Don't throw your hands up in despair quite yet. Try using braces to understand what the parts of the sentence are: "Give the book to {whoever answers the door}." Or, if you're a masochist, you could try diagramming the sentence. However, the King would prefer that the unholy and arcane and diabolical art of diagramming be forgotten forever by everyone. (Yes, yes, you're right: The King never did understand diagramming.)
This confusion doesn't explain all misuses of whomever. The King has heard people use the word as the subject of a sentence, although usually in a way that indicates confusion about what word or clause is the object of the verb. Example: "Whomever is the choice will have a tough job ahead of him." That should be whoever. You could avoid confusion by using dated English syntax and expanding the sentence a bit: "The man whom we choose must endure difficult and strenuous days!" But people, not excluding the King, will look at you oddly.
Unfortunately, it gets trickier. Here are two correct sentences, although you might not think they're correct. (Grammatically, that is. The King is sure you'll agree that the statements are factually correct.) "After January 20, 2001, America was subjected to the administration of a reeking pile of horse manure in the approximate shape of a man whom some citizens mistakenly considered the President." "After January 20, 2001, America was destroyed, debased, and humiliated by an incompetent, reeking pile of horse manure in the rough shape of a man who many Americans were tragically propagandized into thinking was the President."
Why is it whom in the first sentence but who in the second? The King is tempted to mix himself a stiff drink and say, "Because I said so," but he'll try to explain.
Try reordering the sentences. In the first one, the part in question can be rephrased some citizens mistakenly considered whom the President. So whom is the object of the verb considered. In the second sentence, the crucial part is equivalent to many Americans were tragically propagandized into thinking that who was the President. So in this case, who is the subject of the clause who was the President.
Yeah, it's tough. Study hard. Or see the next two paragraphs.
You can play it safe by always using who and whoever. Like it or not - and the King doesn't - the objective forms, whom and whomever, are disappearing from English. So just say, "Who shall I give it to?" Or, "Whoever shall I give it to?" Why, whoever answers the door, of course. Or, "In the early 21st century, a reeking pile of horse manure whose shape bore a rough similarity to that of a man, who certain television newsreaders called the President as often as they could in an effort to brainwash viewers into thinking the manure pile really was the President, brought about the rapid decline of America from Number One Power to historical footnote, after which the manure pile retired to the former pig farm it owned and was never heard of again, although the stench it had generated continued to bedevil America for many more years."
Those who know better about who and whom will sneer, and that includes the King, but once having sneered, he'll sigh and shrug his shoulders and mutter something about these being evil and declining days, and then he'll let it pass. It's that Canute/tide thing again.
Perhaps I can simplify the above.
Too confusing? Okay, as King David wisely suggested, just play it safe and avoid the word whomever. Stick with whoever, just as many people never use the word whom, but only the word who. Believe me, saying whoever when you should say whomever sounds less ignorant than saying whomever when you should say whoever! The latter error is as bad as saying him when you should say he. And I hope you would never say, "Him is going out tonight."
Were I but King of Anglophonia
Hypercorrection
Queen Leonore's Addition
Examples: "You and I are going to eat out tonight." "He and she both enjoy shopping."
It is never correct, even if one is speaking informally, to say something like "Him and me are going to the movies." Think about it this way: He is going to the movies and I am going to the movies. Thus, he and I are going to the movies. We are both subjects of that sentence. To keep this straight, just break up the pronouns in your mind: She enjoys shopping. He enjoys shopping. He and she both enjoy shopping.
King David is correct when he says that the confusion most often arises when the speaker wishes to talk about more than one object. Remember: If they are objects when alone, they are objects when joined. More examples: "They loved both us and him." "They gave them to Tom and us." "The letter was for David and me."
However, Gloria is not going with him, she's not going with me, so she's not going with him and me (with us). Here, him and me are objects of the preposition with. The object of the preposition is the word which follows it. "That's between him and me." Here, the pronouns are the objects of the preposition between.
Examples: "Who is the smirking politician who has failed to carry out the vast majority of his campaign promises?" "Whoever continues to believe in his moral rectitude needs to wake up to the truth before we are saddled with this horrible, dangerous creature for another four years."
Examples: "I greeted whomever I met." "Give the package to whomever (i.e., to anyone at all)." "Go out with whomever you please."
But (careful now!): "Give the package to whoever answers the door" is correct. This is because whoever is now the subject of the clause "whoever answers the door," i.e., the person who answers the door. I trust you would not say, "Give the package to the person whom answers the door."