Grammatical changes

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.[17] Declension was greatly simplified during the Middle English period, when the accusative and dative cases of the pronouns merged into a single oblique case that also replaced the genitive case after prepositions. Nouns in Modern English no longer decline for case, except for the genitive.
 

Evolution of English pronouns


"Who" and "whom", "he" and "him", "she" and "her", etc. are a conflation of the old accusative and dative cases, as well as of the genitive case after prepositions. This conflated form is called the oblique case, or the object (objective) case because it is used for objects of verbs (direct, indirect, or oblique) as well as for objects of prepositions.  The information formerly conveyed by having distinct case forms is now mostly provided by prepositions and word order. In Old English as well as modern German and Icelandic as further examples, these cases had distinct forms.

Although the traditional terms accusative and dative continue to be used by some grammarians, these are roles rather than actual cases in Modern English. That is, the form whom may play accusative or dative roles (or instrumental or prepositional roles), but it is a single morphological form and therefore a single case, contrasting with nominative who and genitive whose. Many grammarians use the more intuitive labels 'subjective', 'objective', and 'possessive' for nominative, oblique, and genitive pronouns.