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Case systems mark grammatical and semantic relationships through morphological forms on nouns and modifiers. Languages vary enormously: English has minimal case (he/him), while Finnish has 15 cases. Cases encode grammatical function (nominative for agents, accusative for patients) and semantic relations (locative, instrumental, comitative). The inventory and functions of cases correlate with word order and other morphosyntactic properties.
Map case systems across several languages, identifying the semantic and grammatical functions of each case. Examine how case systems relate to word-order properties and other morphological systems.
From your study of linguistic typology, you know that languages vary systematically in their morphosyntactic strategies — and case systems are one of the most dramatic sites of this variation. A case system is a set of morphological forms on nouns (and often adjectives and pronouns) that signal the noun's grammatical role or semantic relationship within the clause. The core function is relational: case tells you how a noun phrase relates to the verb and to other noun phrases. English has almost entirely lost this system — only pronouns still reflect it (*he* vs. *him*, *she* vs. *her*) — but this minimal residue shows you what case does: *he saw her* and *her saw he* are formally distinct because the case forms signal who is the subject (agent) and who is the object (patient).
Languages are classified by which distinctions their case systems mark. The most widely attested case is the nominative-accusative alignment: the subject of both transitive and intransitive verbs takes one form (nominative), while the object of a transitive verb takes another (accusative). Latin, German, Russian, and most Indo-European languages with case follow this pattern. A competing pattern is ergative-absolutive alignment, found in Basque, Georgian, and many Australian Aboriginal languages: here, the subject of an intransitive verb and the object of a transitive verb take one form (absolutive), while the subject of a transitive verb (the agent doing something to someone) takes a special form (ergative). The conceptual difference is striking — ergative systems group "affected participants" together rather than grouping "subjects" together as nominative-accusative languages do. Your typological background lets you ask: why would languages converge on this alternative grouping? The answer connects to how languages conceptualize events and participants.
Beyond these core structural cases, languages add semantic cases encoding spatial and other relations that English expresses with prepositions. Finnish, which you may encounter as a benchmark for case-rich systems, has 15 cases including the *inessive* (inside: "in the house"), *elative* (out of: "from the house"), *illative* (into: "into the house"), *adessive* (on/at: "at the house"), *ablative* (away from surface: "from the house's surface"), and *allative* (toward surface: "to the house"). These are not grammatical cases in the nominative-accusative sense — they are essentially postpositions that have been grammaticalized onto the noun, a process your knowledge of grammaticalization mechanisms helps you recognize. This is not random: languages with richer case systems tend to have freer word order, because when case suffixes carry the relational information, word order can vary without ambiguity. The correlation between case richness and word-order freedom is one of the cleaner typological patterns in the literature.
The polyfunctionality of case forms is important to internalize. The Latin *ablative* case serves as an instrument ("with a sword"), an agent in passive constructions ("by Caesar"), a source ("from Rome"), a location ("in the forum"), and a comparison ("better than gold"). These functions are semantically related through a family-resemblance logic — many developed through grammaticalization from more specific spatial meanings — but they are not reducible to a single semantic primitive. This polyfunctionality means that case systems require contextual interpretation; the same suffix can only be correctly parsed given knowledge of the surrounding clause structure and the lexical semantics of the noun and verb involved.