The English language once had an extensive declension system similar to modern German or Icelandic. Old English distinguished between the nominative, accusative, dative, genitive, and instrumental cases. Declension fell into disuse during the Middle English period, when accusative and dative pronouns merged into a single objective pronoun. Modern English no longer uses declension, except for remnants of the former system in a few pronouns.

"Who" and "whom", "he" and "him", "she" and "her", etc. are remnants of both the old nominative vs. accusative and also of nominative vs. dative. In other words, "whom" serves as both the dative and accusative version of the nominative pronoun "who". In Old English (and in modern German, Icelandic, etc.), these cases had distinct pronouns. The word "whom" itself began falling into widespread disuse in the 20th century, and is being replaced by merely "who".

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.

Evolution of English declension

Interrogative pronouns

Old masculine/feminine to the modern person

Old neuter to the modern thing

¹ - Usually replaced by of which, except where it would produce an intolerably clumsy form.

First person personal pronouns

Singular

Plural

Second person personal pronouns

Old and Middle English singular to the Modern English archaic informal

Old and Middle English plural to the archaic formal to the modern general

You in the nominative case was used in Middle English only as a formal but not as a plural pronoun. So there was a difference between You are (singular formal) and Ye are (plural informal).