Some of these issues are not essential to basic communication in English but instead function as "style markers" or "prestige usages" that indicate higher levels of learning and maintain finer distinctions in style. Most people can get along without these distinctions, but writers aren't most folks.
Working at this high a level carries risks. If a speaker attempts the finer points of style and bungles them, the attempt may backfire in terms of lost prestige that wouldn't have been lost if left alone. Even if you manage some of these usages correctly, remember your audience: even if you're right, using them may open an unnecessary gulf between you and regular folks. Always speak and write your best, but always consider what's best for your audience.
Distinguishing between who and whom is a remnant of an earlier phase of the English language. Using "whom" often appears as a sign of learning or education. But even educated speakers bungle the usage so often as to suggest the following advice:
When speaking, forget about "whom" and just say "who." Virtually no one will ever notice if you say "who" when you should have said "whom." On the other hand, if you say "whom," very few will be able to tell whether you used it correctly or not, leaving you stuck with two possible outcomes:
Therefore, in speech just don't bother with whom!—or whomever.
In writing as opposed to speech, however, you can take time to get things right, so the difference between "who" and "whom" can sometimes help, especially if you're writing for a highly educated audience. Knowing how to use "whom" can be a so-called "prestige usage" or "a finer point of style," especially in formal English.
Overall there's a simple way to determine "who vs. whom."
Background: "Who" & "Whom" (+ Whoever & Whomever) are pronouns. More specifically, they can function as either interrogative* or relative* pronouns.
However, the difference between Who & Whom parallels a standard distinction between the most basic and familiar pronouns as they divide to subjective and objective cases.
"Case" is an inflection or form of a noun or pronoun that indicates its grammatical function.
Briefly, "subjective case" and "objective case" divides those pronouns that act as subjects of verbs, and those pronouns that act as objects of verbs or prepositions (of, to, between, etc.).*
cases of personal pronouns
Who, Whom, & Whose correspond respectively to the subjective, objective, and possessive cases above.
Thus the simplest rule for determing who or whom: (All examples below are correct.)
If [WHO/M] can be substituted by HE, SHE, or THEY, then it is WHO.
e.g., "It was Thomas Jefferson, I think, WHO was the third president of the United States." (That is, you would say "HE was the third president of the United States," not "HIM was the third president of the United States."
If [WHO/M] could be substituted by HIM, HER, or THEM, then it is WHOM.
e.g., "It was Thomas Jefferson, I think, WHOM John Adams defeated to become the second president of the United States." (That is, you would say "John Adams defeated HIM," not "John Adams defeated HE.")
"Jones is the man WHO took me fishing last spring." (That is, you would say "HE took me fishing.")
"Jones is the man WHOM I went fishing with last spring." (That is, you would say "I went fishing with HIM.") [Alternative: "Jones is the man with WHOM I went fishing last spring."
A couple of tricky ones—all examples are correct:
Correct: "I gave the tickets to whoever could use them." This might look wrong because "whoever" follows the preposition "to" and so may appear to be the object of a preposition, which would require "whomever." However, the object of the preposition is the entire clause "whoever could use them," where "whoever" is the subject of the verb "could use."
Correct: "I gave the tickets to whomever."
One reason "whom" is so strange and difficult to use correctly: "Whom" (along with other pronouns) is one of only a few synthetic or inflected remnants in modern English, which is primarily an analytic language.
(In a synthetic language like Latin, the function of a word is known by its form; subjects can come after objects without affecting the grammar. In an analytic language like modern English, the function of most words is known by their positions in a clause. Other examples of synthetic language or inflection in English:
One other frequent problem with forms of who:
"Whose," the possessive form of the interrogative or relative pronoun Who, is commonly misspelled as "who's."
Incorrect: "Who's sneakers are these?"
Correct: "Whose sneakers are these?"
Incorrect: "He's a fellow who's friends don't even trust him."
Correct: "He's a fellow whose friends don't even trust him."
Why? Apostrophe + s ('s) is used to make nouns possessive, so some people think it works for pronouns too, but who's is like it's.
Just as "it's" can only be a contraction for "it is," so "who's" can only be a contraction for "who is."
Correct: Who's driving me to work? (who = interrogative pronoun)
Correct: There's the man who's driving me to work. (who = relative pronoun in a relative clause)
Style Marker: Writing or saying "that" for "who"
It is grammatically correct to use "that" for "who" in a relative clause but stylistically incorrect:
Grammatically correct but stylistically inferior: “There's the man that took my newspaper”
Grammatically correct and stylistically superior: : “There's the man who took my newspaper.”
Ninety percent of English speakers can't tell the difference, but the ten percent who can tend to be the ones who make decisions about language.
The main reason this distinction is worth keeping is because "that" is supposed to be reserved for non-human references, as in "I loaded all the cattle that would fit in the truck."
If you say, "I'm here to serve all the people that vote for me," you're addressing people as though they're not people but animals or things.
Examples (correct) from GrammarBook.com:
GrammarGirl.com: "The quick and dirty answer is that you use who when you are talking about a person and that when you are talking about an object. Stick with that rule and you'll be safe. . . . I always think of my friend who would refer to his new stepmother only as the woman that married my father. He was clearly indicating his animosity and you wouldn't want to do that accidentally."
Charles Ray for DailyWritingTips.com:
Some might think me an old fashioned, stuffy person when it comes to grammar. I realize that language, whether written or spoken, is a living thing, and that it changes with time, but there are some modern conventions that I have problems with. . . . One of the modern conventions is the use of ‘that’ in sentences when logic, and my ear, tells me that ‘who’ would be more appropriate. Here, for instance, is a sentence I encountered recently in a paper written by a college graduate: “The judge that decided the case came from the lower court.” Now, I assume the judge in this sentence is human, and when I struggled with English grammar many decades ago, I would have been told in no uncertain terms that the correct formulation is, “The judge who . . . .”
I have been chided by many of my colleagues for my fussiness over this particular issue . . . . They’ve pointed out that this is not ‘incorrect,’ and besides, it has become accepted usage among a large number of writers. Well, not this writer. Correct, it might be, and I’m not entirely convinced of that, but it just doesn’t sound right. . . . The language and its grammar are constantly changing, but this is no reason for us to blithely accept each change."
Exceptions: This distinction is fairly recent in the English language. "That" often served as "who" in earlier forms of English and still sounds acceptable when it refers to humans less personally or more as types or representatives.
Examples of exceptions:
"I regarded my friend as the most impartial critic that could be found."
"They are the sort of characters that one does one's best to avoid."
SentenceSleuth.com: "My grammar sources tell me that Shakespeare and the Bible, for example, use 'that' after a person. So although it isn't incorrect to use 'that' after a person, it's more common to use 'who,' and I prefer 'who.'"
While discussing subjective and objective cases, Mrs. White is keen on "the educated person's mistake," for instance: (next two examples are incorrect):
Both these usages are wrong and should say "me" instead of "I." Nearly any English speaker can tell they're wrong because "I" sounds wrong and "me" sounds normal.
So where does the problem come from? Such hypercorrectness may extend from occasional situations where rules outweigh common sense. In the past, for instance, many students were taught not to say "It's me," which is technically incorrect but sounds right, but instead "It is I," which is technically correct but sounds wrong.
Since most people don't understand rules or processes of language anyway, they start applying the effect rather than the process. Therefore, if "I" doesn't sound right but is right in one place, maybe saying something that sounds wrong gains you prestige in another situation.
The only rule to keep in mind is to speak both as well and as naturally as you can, and don't get hung up on rules unless you have time to work them out on paper.
But while we're at it, here are the rules in the cases cited.
"It is I" or "It is he" is technically correct because the case of the pronoun is subjective or nominative; i.e., I, we, he, she, they, who, etc. The reason it is subjective or nominative is because "is" is not a transitive verb but a copula or linking verb. So even though "I" or "he" is at the end of a clause like an object would normally be (as in "It hit me," with hit as a transitive verb), technically the sentence could be reversed: "I am it," though you'd probably say something more like "I'm here."
However, in the cases of "call Freddy or I," or "Between you and I," these pronouns should be objective case because they are objects of the transitive verb "call" or the preposition "between."
Correct: "call Freddy or me"; "between you and me."
* * * * * * *
*interrogative pronouns: In linguistics, an interrogative word is a function-word used to ask questions, such as what, when, where, who, why and how. The interrogative pronouns are who & whom (personal) and what and which (impersonal).
*relative pronouns: A relative pronoun is a pronoun that marks a relative clause and has the same referent as the element of the main clause (usu. a noun or noun phrase) that the relative clause modifies.
The relative pronouns are that, which, who, and whom.
Examples of relative pronouns and clauses:
Brian said goodnight to his roommate Justin, who continued to play video games until his eyes grew blurry with fatigue.
("Who" is a relative pronoun that renames "Justin." The relative clause—"who continued to play . . . "—modifies Justin.
"Peace is not merely a distant goal that we seek, but a means by which we arrive at that goal."—Dr. Martin Luther King, Jr.
("that we seek" and "by which we arrive . . . " are relative clauses. "That" renames "goal"; "which" renames "means.")
"The essence of childhood, of course, is play, which my
friends and I did endlessly on streets that we reluctantly shared with
*prepositions: in the following examples, on, across, down, to, in—plus many others (e.g., under, above, outside, before, etc.)
(All the highlighted words are prepositional phrases.)
Following websites were consulted especially for examples and quotations: http://www.examiner.com/article/the-battle-between-who-and-whom-quiz, http://web.ku.edu/~edit/whom.html, http://grammar.quickanddirtytips.com/who-versus-that.aspx, http://www.grammarbook.com/grammar/whoVwhVt.asp, and http://www.dailywritingtips.com/who-or-that-%E2%80%93-that-is-the-question/.