The English language once had an extensive declension system similar to Latin, modern German or Icelandic. Old English distinguished between the nominative, accusative, dative, and genitive cases; and for strongly declined adjectives and some pronouns also a separate instrumental case (which otherwise and later completely coincided with the dative). In addition, the dual was distinguished from the more modern singular and plural. Declension was greatly simplified during the Middle English period, when accusative and dative pronouns merged into a single objective pronoun. Nouns in Modern English no longer decline for case, except in a sense for possessive, and 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 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.