Rick Morneau, in The Lexical Semantics of a Machine Interlingua, distinguishes several case roles. A given language might mark some case roles with case inflection, some with prepositions (or postpositions), and some with word order. The most common case roles include:
- Agent — someone who is actively doing the action denoted by the verb
- Patient — someone or something that the verb's action is being done to
- Experiencer — someone who is experiencing the process or state denoted by the verb
- Focus (also called theme, object, or topic) — something that is focused on by the verb's action, but is not really affected by it
- Recipient — the indirect object of a ditransitive verb, like "to give something to someone".
Few if any natural languages, and relatively few conlangs, have one case inflection or adposition for each of these roles. Most tend to bundle two or three of them into one syntactic or morphological case. Languages can be divided into types based on how they divide up these semantic roles into cases, and how they are marked:
The classification is done by looking at how languages treat these syntactic roles:
- S, the one noun associated with an intransitive verb ("The cat sleeps", "The jogger runs")
- A, the agent noun associated with a transitive verb
- P, the patient or focus noun associated with a transitive verb
Nominative-accusative languages like English, Greek, and so forth (most Indo-European languages, and many others) use the same case inflection or word order position to mark S as A (nominative case in Greek, position before the verb in English), and treat P differently (accusative case as in Greek, the object position after the verb as in English). Ergative-absolutive languages, like Basque, treat S like P. Active languages treat S sometimes like A and sometimes like P, depending on various criteria. Tripartite languages treat S, A and P all differently. Trigger languages are treated in the advanced section.