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The pronoun '''who''', in the English language, is the interrogative and relative pronoun that is used to refer to human beings.
The corresponding interrogative pronouns for non-sentient beings are ''what'' and ''which'', and the relative pronouns are ''that'' and ''which''. ''That'' and ''which'' are sometimes used in contexts where ''who'' might be a more suitable choice, and ''who'' is likewise used in contexts where ''that'' or ''which'' would be a more suitable choice.
In etymology, the spelling represents the expected outcome of Old English ''hwâ'', while the actual pronunciation represents a divergent outcome. It is cognate with Latin ''quis'' and Greek ''ποιός''.

Contents

Case forms

Rules for determining ''who'' vs. ''whom'' in traditional usage

Use with prepositions

Indirect object with or without ''to''

Who(m)(so)ever

Subject ''whom''

References

Case forms

Traditionally, ''who'' is the subjective (nominative) form only. According to traditional Prescriptive grammar: ''who'' is a subjective pronoun (subject of the side clause), and ''whom'' is the corresponding dative pronoun (an object of the side clause). ''Whose'' is the possessive form, which is sometimes confused with ''who's'', a contraction of ''who is''. See also: ''English declension''.

★ '' He is someone 'to whom' I owe a great deal.'' ("I" is subject, "whom" (relating to "he") the object)

★ '' He is someone 'who' is a great guy.'' ("who" is subject of the subordinate clause)

★ ''He is someone 'whose' help I appreciate.'' ("whose" is adjunct to help, the side clause's subject)
The form ''who'' increasingly replaces ''whom'' in objective case contexts. According to the OED (2nd edition, 1989), ''whom'' is "no longer current in natural colloquial speech". Lasnik and Sobin (2000) argue that surviving occurrences of ''whom'' are not part of ordinary English grammar, but the result of extra-grammatical rules for producing "prestige" forms.
''Whom'' remains in significant use following a preposition (see examples immediately below). In informal contexts, the preposition may instead be placed at the end (see ''preposition stranding''), and the word ''who'' may be omitted where it is used as a relative pronoun. For example:

★ (relative, formal): ''He is someone to whom I owe a great deal.''

★ (interrogative, formal): ''To whom did you give it?''

★ (relative, informal): ''He is someone (who) I owe a great deal to.''

★ (interrogative, informal): ''Who(m) did you give it to?''

Rules for determining ''who'' vs. ''whom'' in traditional usage

Use with prepositions

''Whom'' is the form used when prepositions are involved. Again this is analogous to personal pronouns, for which the objective form is also used after a preposition. For example:

★ ''To whom have you been talking?'' (Compare: ''You have been talking 'to him'''.)

★ ''For whom have you taken these marvellous photographs?'' (''You have taken these marvellous photographs 'for him'.'')

★ ''With whom are you going to the cinema?'' (''You are going to the cinema 'with him'.'')

★ ''He sent gifts to his granddaughter, of whom he was fond.''

★ ''He sent gifts to his granddaughter, whom he was fond of.''
Forms with ''who'' in which the preposition does not immediately precede the pronoun are commonly judged acceptable in everyday use, and in spoken use especially:

★ ''He sent gifts to his granddaughter, who he was fond of.''

Indirect object with or without ''to''

The indirect object is normally formed with ''to'', so is generally just a particular example of the usage with prepositions (''to whom''). In some situations, ''whom'' can also be used by itself to represent the indirect object. However this is only really used in contexts where the direct object is omitted:

★ ''Whom will you tell?''
In fact, the following is normally considered acceptable in everyday use, especially in speech:

★ ''Who will you tell?''
Where the direct object is expressed, ''to'' is generally included, even where an analogous sentence using a personal pronoun might use the pronoun as the indirect object:

★ ''You will tell him a story.''
But:

★ ''To whom will you tell a story?''
When the ''to'' does not immediately precede the pronoun, either ''who'' or ''whom'' is generally considered acceptable:

★ ''Whom will you tell a story to?'' [Rather formal.]

★ ''Who will you tell a story to?'' [Less formal, and more common in fact.]

Who(m)(so)ever

The distributive pronouns ''who(m)ever'' and ''who(m)soever'' take the case appropriate to their internal clause. For example, in

★ ''Let whoever is without sin cast the first stone''
''whoever'' is in the nominative because it is the subject of ''is'' (cf. ''He is without sin''), even though it is also the object of ''let'' (cf. ''Let him cast the first stone''). However, in

★ ''Whomever you meet there is bound to be interesting''
''whomever'' is in the accusative because it is the object of ''meet'' (cf. ''You meet her''), even though it is also the subject of ''is'' (cf. ''She is bound to be interesting''.)

Subject ''whom''

A special problem arises in constructions like this:

Beethoven, who you say was a great composer, wrote only one opera.

The form given with ''who'' is safe, and even beyond reproach (since ''who'' is the subject of ''was''). But many use and defend ''whom'' in such a sentence. The use of ''whom'' may arise from confusion with a form like this, in which ''whom'' is used according to the standard rules:

Beethoven, whom you consider a great composer, wrote only one opera.

Here is an example from ''The Age'' newspaper (Melbourne, Australia), which in April 1999 syndicated an article from the ''Washington Post'' concerning the Columbine massacre:

But if others were involved, it was Harris and Klebold whom students said seemed the tightest, who stood apart from the rest of their clique.

The ''Washington Post's'' original article "correctly" had ''who students said'', but ''The Age'' altered this as a matter of house style to ''whom students said''. (The continuation with the parallel construction '''who' stood apart'' illustrates how ''The Age'' style can lead to inconsistencies.)
Even the King James Bible has the problematic ''whom'' as a subject at least six times, and has been much censured for it:

He saith unto them, But whom say ye that I am? (Matthew 16:15; cf. Matthew 16:13, Mark 8:27,29, Luke 9:18,20)

(Technically, ''ye'' is the subject associated with the verb ''say'' in ''But whom say ye that I am?'', and ''I'' is the subject associated with the verb ''am''. ''Who[m]'' is a subject complement, as it would be in ''Who am I?'' or ''I am who?'' It is not an object complement.)
There are similar examples in Shakespeare:

Young Ferdinand, whom they suppose is drown'd, [...] (''The Tempest'', III, 3)

[...] going to seek the grave / Of Arthur, whom they say is kill'd to-night / On your suggestion. (''King John'', IV, 2)

Daniel Defoe also uses subject ''whom'', here apparently affected by the proximity of ''him'':

They told me that when they were so knocked down, the rest of their company rescued them, and stood over them fighting till they were come to themselves, all but him whom they thought had been dead;[...] (''The Further Adventures of Robinson Crusoe'', Chapter 6, Part 1)

In this case, ''who[m]'' is the subject of ''had been dead''.
The OED traces subject ''whom'' back to the 15th century, while Jespersen (1965 [1924], appendix) cites even earlier examples from Chaucer.

References

★ The Philosophy of Grammar, , Otto, Jespersen, Norton, 1965,

★ The ''who/whom'' puzzle: On the preservation of an archaic feature, , Howard, Lasnik, Natural Language and Linguistic Theory, 2000