A grammatical case is a category of nouns and noun modifiers (determiners, adjectives, participles, and numerals), which corresponds to one or more potential grammatical functions for a nominal group in a wording.[1] In various languages, nominal groups consisting of a noun and its modifiers belong to one of a few such categories. For instance, in English, one says I see them and they see me: the nominative pronouns I/they represent the perceiver and the accusative pronouns me/them represent the phenomenon perceived. Here, nominative and accusative are cases, that is, categories of pronouns corresponding to the functions they have in representation.

English has largely lost its inflected case system but personal pronouns still have three cases, which are simplified forms of the nominative, accusative and genitive cases. They are used with personal pronouns: subjective case (I, you, he, she, it, we, they, who, whoever), objective case (me, you, him, her, it, us, them, whom, whomever) and possessive case (my, mine; your, yours; his; her, hers; its; our, ours; their, theirs; whose; whosever).[2][3] Forms such as I, he and we are used for the subject ("I kicked the ball"), and forms such as me, him and us are used for the object ("John kicked me").

As a language evolves, cases can merge (for instance, in Ancient Greek, the locative case merged with the dative case), a phenomenon officially called syncretism.[4]

Languages such as Latin, Tamil, Russian and German have extensive case systems, with nouns, pronouns, adjectives, and determiners all inflecting (usually by means of different suffixes) to indicate their case. The number of cases differs between languages: Persian has two; modern English has three but for pronouns only; Torlakian dialects, Classical and Modern Standard Arabic have three; German, Icelandic, Modern Greek, and Irish have four; Romanian and Ancient Greek have five; Bengali, Latin, Russian, Slovak, Slovenian, and Turkish each have at least six; Armenian, Czech, Georgian, Kajkavian, Latvian, Lithuanian, Polish, Serbian, Croatian and Ukrainian have seven; Mongolian, Sanskrit, Tamil, and Greenlandic have eight; Assamese has 10; Basque has 13; Estonian has 14; Finnish has 15; Hungarian has 18 and Tsez has 64 cases.[citation needed]

Commonly encountered cases include nominative, accusative, dative and genitive. A role that one of those languages marks by case is often marked in English with a preposition. For example, the English prepositional phrase with (his) foot (as in "John kicked the ball with his foot") might be rendered in Russian using a single noun in the instrumental case, or in Ancient Greek as τῷ ποδί (tôi podí, meaning "the foot") with both words (the definite article, and the noun πούς (poús) "foot") changing to dative form.

More formally, case has been defined as "a system of marking dependent nouns for the type of relationship they bear to their heads".[5]:p.1 Cases should be distinguished from thematic roles such as agent and patient. They are often closely related, and in languages such as Latin, several thematic roles are realised by a somewhat fixed case for deponent verbs, but cases are a syntagmatic/phrasal category, and thematic roles are the function of a syntagma/phrase in a larger structure. Languages having cases often exhibit free word order, as thematic roles are not required to be marked by position in the sentence.