Evolution of English pronouns

"Who" and "whom", "he" and "him", "she" and "her", etc. are remnants of both the old nominative versus accusative and also of nominative versus dative. In other words, "her" (for example) serves as both the dative and accusative version of the nominative pronoun "she". In Old English as well as modern German and Icelandic as further examples, these cases had distinct pronouns.
This collapse of the separate case pronouns into the same word is one of the reasons grammarians consider the dative and accusative cases to be extinct in English — neither is an ideal term for the role played by "whom". Instead, the term objective is often used; that is, "whom" is a generic objective pronoun which can describe either a direct or an indirect object. The nominative case, "who", is called simply the subjective. The information formerly conveyed by having distinct case forms is now mostly provided by prepositions and word order.
Modern English morphologically distinguishes only one case, the possessive case — which some linguists argue is not a case at all, but a clitic (see the entry for genitive case for more information). With only a few pronominal exceptions, the objective and subjective always have the same form.

Interrogative pronouns

	Case	Old English	Middle English	Modern English
Masculine/Feminine (Person)	Nominative	hwā	who	who
	Accusative	hwone / hwæne	whom	who / whom1
	Dative	hwām / hwǣm
	Instrumental
	Genitive	hwæs	whos	whose
Neuter (Thing)	Nominative	hwæt	what	what
	Accusative	hwæt	what / whom
	Dative	hwām / hwǣm
	Instrumental	hwȳ / hwon	why	why
	Genitive	hwæs	whos	whose2

¹ - In some dialects who is used where Formal English only allows whom, though variation among dialects must be taken into account.
² - Usually replaced by of what (postpositioned).

First person personal pronouns

	Case	Old English	Middle English	Modern English
Singular	Nominative	iċ	I / ich / ik	I
	Accusative	mē / meċ	me	me
	Dative	mē
	Genitive	mīn	min / mi	my, mine
Plural	Nominative	wē	we	we
	Accusative	ūs / ūsiċ	us	us
	Dative	ūs
	Genitive	ūser / ūre	ure / our	our, ours

(Old English also had a separate dual, wit ("we two") etcetera; however, no later forms derive from it.)

Second person personal pronouns

Old and Middle English singular to the Modern English archaic informal

	Case	Old English	Middle English	Modern English
Singular	Nominative	þū	þu / thou	thou (you)
	Accusative	þē / þeċ	þé / thee	thee (you)
	Dative	þē
	Genitive	þīn	þi / þīn / þīne / thy /thin / thine	thy, thine (your)
Plural	Nominative	ġē	ye / ȝe / you	you
	Accusative	ēow / ēowiċ	you, ya
	Dative	ēow
	Genitive	ēower	your	your, yours

Note that the ye/you distinction still existed, at least optionally, in Early Modern English: "Ye shall know the truth and the truth shall make you free" from the King James Bible.
Here the letter þ (interchangeable with ð in manuscripts) corresponds to th.

Formal and informal forms of the second person singular and plural

Old English

Middle English

Modern English

Singular

Plural

Singular

Plural

Singular

Plural

Case

Formal

Informal

Formal

Informal

Formal

Informal

Formal

Informal

Formal

Informal

Formal

Informal

Nominative

þū

ġē

you

thou

you

ye

you

Accusative

þē / þeċ

ēow / ēowiċ

thee

you

Dative

þē

ēow

Genitive

þīn

ēower

your, yours

thy, thine

your, yours

your, yours

(Old English also had a separate dual, ȝit ("ye two") etcetera; however, no later forms derive from it.)

Third person personal pronouns

	Case	Old English	Middle English	Modern English
Masculine Singular	Nominative	hē	he	he
	Accusative	hine	him	him
	Dative	him
	Genitive	his	his	his
Feminine Singular	Nominative	hēo	heo / sche / ho / he / ȝho	she
	Accusative	hīe	hire / hure / her / heore	her
	Dative	hire
	Genitive	hire	hir / hire / heore / her / here	her, hers
Neuter Singular	Nominative	hit	hit / it	it
	Accusative	hit	hit / it / him
	Dative	him
	Genitive	his	his / its	its
Plural	Nominative	hīe	he / hi / ho / hie / þai / þei	they
	Accusative	hīe	hem / ham / heom / þaim / þem / þam	them
	Dative	him
	Genitive	hira	here / heore / hore / þair / þar	their, theirs

(The origin of the modern forms is generally thought to have been a borrowing from Old Norse forms þæir, þæim, þæira.
The two different roots co-existed for some time, although currently the only common remnant is the shortened form 'em.
Cf. also the demonstrative pronouns.)