Case is a special grammatical category of a noun, pronoun, adjective, participle or numeral whose value reflects the grammatical function performed by that word in a phrase, clause or sentence. In some languages, nouns, pronouns, adjectives, determiners, participles, prepositions, numerals, articles and their modifiers take different inflected forms, depending on their case. As a language evolves, cases can merge (for instance, in Ancient Greek, the locative case merged with the dative case), a phenomenon formally called syncretism.^[2]

English has largely lost its inflected case system although 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^[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").

Languages such as Ancient Greek, Armenian, Assamese, most Balto-Slavic languages, Basque, most Caucasian languages, German, Icelandic, Japanese, Korean, Latin, Sanskrit, Tamil, Tibetan (one of a few tonal languages), the Turkic languages and the Uralic languages 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: Esperanto has two; modern English has three but for pronouns only; German and Icelandic have four; Romanian has five; Latin, Russian and Turkish each have at least six; Armenian, Czech, Latvian, Lithuanian, Polish, Serbian, Croatian, Slovak and Ukrainian have seven; Sanskrit and Tamil have eight; Estonian has 14; Finnish has 15; Hungarian has 18 and Tsez has 64 cases.

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".^{[4] []} 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 have an associated case, but cases are a morphological notion, and thematic roles a semantic one. Languages having cases often exhibit free word order, as thematic roles are not required to be marked by position in the sentence.

It is widely accepted that the Ancient Greeks had a certain idea of the forms of a name in their own language. A fragment of Anacreon seems to prove this. Nevertheless, it cannot be inferred that the Ancient Greeks really knew what grammatical cases were. Grammatical cases were first recognized by the Stoics and from some philosophers of the Peripatetic school.^[5][6] The advancements of those philosophers were later employed by the philologists of the Alexandrian school.^[7][5]

The English word case used in this sense comes from the Latin casus, which is derived from the verb cadere, "to fall", from the Proto-Indo-European root **ḱad-.^[8] The Latin word is a calque of the Greek πτῶσις, ptôsis, lit. "falling, fall".^[9] The sense is that all other cases are considered to have "fallen" away from the nominative. This imagery is also reflected in the word declension, from Latin declinere, "to lean", from the PIE root **ḱley-.

The equivalent to "case" in several other European languages also derives from casus, including cas in French, caso in Spanish and Kasus in German. The Russian word паде́ж (padyézh) is a calque from Greek and similarly contains a root meaning "fall", and the German Fall and Czech pád simply mean "fall", and are used for both the concept of grammatical case and to refer to physical falls. The Finnish equivalent is sija, whose main meaning is "position" or "place".

Although not very prominent in modern English, cases featured much more saliently in Old English and other ancient Indo-European languages, such as Latin, Old Persian, Ancient Greek, and Sanskrit. Historically, the Indo-European languages had eight morphological cases, though modern languages typically have fewer, using prepositions and word order to convey information that had previously been conveyed using distinct noun forms. Among modern languages, cases still feature prominently in most of the Balto-Slavic languages (except Macedonian and Bulgarian^[10]), with most having six to eight cases, as well as Icelandic, German and Modern Greek, which have four. In German, cases are mostly marked on articles and adjectives, and less so on nouns. In Icelandic, articles, adjectives, personal names and nouns are all marked for case, making it, among other things, the living Germanic language that could be said to most closely resemble Proto-Germanic.

The eight historical Indo-European cases are as follows, with examples either of the English case or of the English syntactic alternative to case:

All of the above are just rough descriptions; the precise distinctions vary significantly from language to language, and as such they are often more complex. Case is based fundamentally on changes to the noun to indicate the noun's role in the sentence – one of the defining features of so-called fusional languages. Old English was a fusional language, but Modern English does not work this way.

Modern English has largely abandoned the inflectional case system of Proto-Indo-European in favor of analytic constructions. The personal pronouns of Modern English retain morphological case more strongly than any other word class (a remnant of the more extensive case system of Old English). For other pronouns, and all nouns, adjectives, and articles, grammatical function is indicated only by word order, by prepositions, and by the "Saxon genitive" (-'s).^[1]

Taken as a whole, English personal pronouns are typically said to have three morphological cases:

Most English personal pronouns have five forms: the nominative and oblique case forms, the possessive case, which has both a determiner form (such as my, our) and a distinct independent form (such as mine, ours) (with two exceptions: the third person singular masculine and the third person singular neuter it, which use the same form for both determiner and independent [his car, it is his]), and a distinct reflexive or intensive form (such as myself, ourselves). The interrogative personal pronoun who exhibits the greatest diversity of forms within the modern English pronoun system, having definite nominative, oblique, and genitive forms (who, whom, whose) and equivalently coordinating indefinite forms (whoever, whomever, and whosever).

Though English pronouns can have subject and object forms (he/him, she/her), nouns show only a singular/plural and a possessive/non-possessive distinction (e.g. chair, chairs, chair's, chairs'). Note that chair does not change form between "the chair is here" (subject) and "I saw the chair" (direct object), a distinction made by word order and context.

Cases can be ranked in the following hierarchy, where a language that does not have a given case will tend not to have any cases to the right of the missing case:^{[4] []}

This is, however, only a general tendency. Many forms of Central German, such as Colognian and Luxembourgish, have a dative case but lack a genitive. In Irish nouns, the nominative and accusative have fallen together, whereas the dative–locative has remained separate in some paradigms; Irish also has genitive and vocative cases. In Punjabi, the accusative, genitive, and dative have merged to an oblique case, but the language still retains vocative, locative, and ablative cases. Old English had an instrumental case, but not a locative or prepositional.

The traditional case order (nom-gen-dat-acc) was expressed for the first time in The Art of Grammar in the 2nd century BC:

The Russian language uses a similar case order.(ru:Русский язык#Имя существительное)

Latin grammars, such as Ars grammatica, followed the Greek tradition, but added the ablative case of Latin. Later other European languages also followed that Graeco-Roman tradition.

However, for some languages, such as Latin, due to case syncretism the order may be changed for convenience, where the accusative or the vocative cases are placed after the nominative and before the genitive. For example:

In the most common^[4] case concord system, only the head-word (the noun) in a phrase is marked for case. This system appears in many Papuan languages as well as in Turkic, Mongolian, Quechua, Dravidian, Indo-Aryan, and other languages. In Basque and various Amazonian and Australian languages, only the phrase-final word (not necessarily the noun) is marked for case. In many Indo-European, Finnic, and Semitic languages, case is marked on the noun, the determiner, and usually the adjective. Other systems are less common. In some languages, there is double-marking of a word as both genitive (to indicate semantic role) and another case such as accusative (to establish concord with the head noun).^[18]

Declension is the process or result of altering nouns to the correct grammatical cases. Languages with rich nominal inflection (use grammatical cases for many purposes) typically have a number of identifiable declension classes, or groups of nouns with a similar pattern of case inflection or declension. Sanskrit has six declension classes, whereas Latin is traditionally considered to have five, and Ancient Greek three declension classes.^[19] For example, Slovak has fifteen noun declension classes, five for each gender (the number may vary depending on which paradigms are counted or omitted, this mainly concerns those that modify declension of foreign words; refer to article).

In Indo-European languages, declension patterns may depend on a variety of factors, such as gender, number, phonological environment, and irregular historical factors. Pronouns sometimes have separate paradigms. In some languages, particularly Slavic languages, a case may contain different groups of endings depending on whether the word is a noun or an adjective. A single case may contain many different endings, some of which may even be derived from different roots. For example, in Polish, the genitive case has -a, -u, -ów, -i/-y, -e- for nouns, and -ego, -ej, -ich/-ych for adjectives. To a lesser extent, a noun's animacy or humanness may add another layer of complexity. For example, in Russian:

vs.

and

Assamese has ten cases.

An example of a Belarusian case inflection is given below, using the singular forms of the Belarusian term for "country," which belongs to Belarusian's first declension class.

In German, grammatical case is largely preserved in the articles and adjectives, but nouns have lost many of their original endings. Below is an example of case inflection in German using the masculine definite article and one of the German words for "sailor".

An example with the feminine definite article with the German word for "woman."

An example with the neuter definite article with the German word for "book."

Modern Greek has four cases: nominative, genitive, accusative, and vocative. For neuters and most groups of feminines and plural masculines, the genitive case differs from the other three. Below is an example of the declension of ουρανός (sky), which has a different form in the singular of all four cases, together with the appropriate article in both the singular and the plural:

Ancient Greek had one additional case, the dative. At some point, it was replaced with the preposition εις, followed by the accusative. This became necessary when pronunciation simplified, merging the two long vowels eta and omega to short. The result was that dative did not sound much different from the accusative in the singular of the first two groups. However, the dative case is still used in many expressions.

With time, only the sigma of εις was left and got attached to the article, except when an article is not used and it becomes σε instead. Note that this is not a different case from the accusative.

Below is an example with the dative case of the word πόλη (city):

Cases in Japanese are marked by particles placed after the nouns.^[20] A distinctive feature of Japanese is the presence of two cases, which are roughly equivalent to the nominative case in other languages: one representing the sentence topic, the other representing the subject. The most important case markers are the following:

Cases in Korean are marked by particles placed after the nouns, similar to Japanese. Like Japanese, the nominative case has two distinctions, one representing the topic of a sentence and the other the subject. In informal speech, nominative (이/가, 께서, and 에서) and accusative (을/를) particles are often omitted, while dative (에게) and ablative (에서) are shortened to simply 에, if the meaning of the sentence can easily be inferred from context. Most common case markers are the following:

An example of a Latin case inflection is given below, using the singular forms of the Latin term for "cook," which belongs to Latin's second declension class.

Latvian nouns have seven grammatical cases: nominative, genitive, dative, accusative, instrumental, locative and vocative. The instrumental case is always identical to the accusative in the singular and to the dative in the plural. It is used as a free-standing case (without a preposition) only in highly restricted contexts in modern Latvian.

An example of a Latvian case inflection is given below, using the singular forms of the Latvian term for "man," which belongs to the first declension class.

In Lithuanian, only the inflection usually changes in the seven different grammatical cases:

Vocative forms are given in parentheses after the nominative, as the only pronominal vocatives that are used are the third person ones, which only occur in compounds^[21].

An example of a Polish case inflection is given below, using the singular forms of the Polish terms for "human" (człowiek) and "monkey" (małpa)

Hungarian declension is relatively simple with regular suffixes attached to the vast majority of nouns. The following table lists a few of the many cases used in Hungarian.

Romanian is the only modern major Romance language with a case system for all nouns, whereas all other Romance languages dropped the cases for nouns replacing them by prepositions. An example of Romanian case inflection is given below, using the singular form of the word "boy":

An example of a Russian case inflection is given below (with explicit stress marks), using the singular forms of the Russian term for "sailor," which belongs to Russian's first declension class.

Up to ten additional cases are identified by linguists, although today all of them are either incomplete (do not apply to all nouns or do not form full word paradigm with all combinations of gender and number) or degenerate (appear identical to one of the main six cases). The most recognized additional cases are locative (в лесу́, на мосту́, в слеза́х), partitive (ча́ю, са́хару, песку́), and two forms of vocative — old (Го́споди, Бо́же, о́тче) and neo-vocative (Маш, пап, ребя́т). Sometimes, so called count-form (for some countable nouns after numerals) is considered to be a sub-case. See details.

Grammatical case was analyzed extensively in Sanskrit. The grammarian Pāṇini identified six semantic roles or kāraka,^[22] which by default are related to the following eight Sanskrit cases in order:^[23]

For example, in the following sentence leaf is the agent (kartā, nominative case), tree is the source (apādāna, ablative case), and ground is the locus (adhikaraṇa, locative case). The declensions are reflected in the morphemes -āt, -am, and -au respectively.

However, the cases may be deployed for other than the default thematic roles. A notable example is the passive construction. In the following sentence, Devadatta is the kartā, but appears in the instrumental case, and rice, the karman, object, is in the nominative case (as subject of the verb). The declensions are reflected in the morphemes -ena and -am.

The Tamil case system is analyzed in native and missionary grammars as consisting of a finite number of cases.^[24][25] The usual treatment of Tamil case (Arden 1942)^[26] is one in which there are seven cases: nominative (first case), accusative (second case), instrumental (third), dative (fourth), ablative (fifth), genitive (sixth), and locative (seventh). In traditional analyses, there is always a clear distinction made between post-positional morphemes and case endings. The vocative is sometimes given a place in the case system as an eighth case, but vocative forms do not participate in usual morphophonemic alternations and do not govern the use of any postpositions. Modern grammarians, however, argue that this eight-case classification is coarse and artificial^[25] and that Tamil usage is best understood if each suffix or combination of suffixes is seen as marking a separate case.^[27]

Telugu has eight cases.

As languages evolve, case systems change. In early Ancient Greek, for example, the genitive and ablative cases became combined, giving five cases, rather than the six retained in Latin. In modern Hindi, the Sanskrit cases have been reduced to three: a direct case (for subjects and direct objects) and oblique case, and a vocative case.^[28][29] In English, apart from the pronouns discussed above, case has vanished altogether except for the possessive/non-possessive dichotomy in nouns.

The evolution of the treatment of case relationships can be circular.^{[4] []} Adpositions can become unstressed and sound like they are an unstressed syllable of a neighboring word. A postposition can thus merge into the stem of a head noun, developing various forms depending on the phonological shape of the stem. Affixes can then be subject to various phonological processes such as assimilation, vowel centering to the schwa, phoneme loss, and fusion, and these processes can reduce or even eliminate the distinctions between cases. Languages can then compensate for the resulting loss of function by creating adpositions, thus coming full circle.

Recent experiments in agent-based modeling have shown how case systems can emerge and evolve in a population of language users.^[30] The experiments demonstrate that language users may introduce new case markers to reduce the cognitive effort required for semantic interpretation, hence facilitating communication through language. Case markers then become generalized through analogical reasoning and reuse.

Languages are categorized into several case systems, based on their morphosyntactic alignment—how they group verb agents and patients into cases:

The following are systems that some languages use to mark case instead of, or in addition to, declension:

The lemma form of words, which is the form chosen by convention as the canonical form of a word, is usually the most unmarked or basic case, which is typically the nominative, trigger, or absolutive case, whichever a language may have.