The term "unaccusative" is applied primarily to nominative-accusative languages, where the accusative case, which marks the direct object of a transitive verb, typically marks the non-volitional role. Unaccusative verbs are so called because their non-volitional arguments do not take the accusative case.
The derivation of the core properties of unaccusative constructions from a set of principles is one of the topmost issues of the agenda of modern syntax since the seminal work by Perlmutter 1978 (cf. Burzio 1986 and Hale-Keyser 2003 for landmark proposals). More specifically, the first approach arrived at an important consequence constituting an analogy between English passive voice constructions and unaccusative constructions whereas in the second approach a more radical theory was proposed based on the analysis of expletive there stemming from the sentences with the copula suggested in Moro 1997.
As mentioned above, the unaccusative/unergative split in intransitive verbs can be characterized semantically. Unaccusative verbs tend to express a telic and dynamic change of state or location, while unergative verbs tend to express an agentive activity (not involving directed movement). While these properties define the "core" classes of unaccusatives and unergatives, there are intermediate classes of verbs whose status is less clear (for example, verbs of existence, appearance, or continuation, verbs denoting uncontrolled processes, or motion verbs).
A number of syntactic criteria for unaccusativity have also been identified. The most well-known test is auxiliary selection in languages that use two different temporal auxiliaries (have and be) for analytic past/perfect verb forms (e.g. German, Dutch, French, Italian). In these languages, unaccusative verbs combine with be, while unergative verbs combine with have.
From one language to another, however, synonymous verbs do not always select the same auxiliary, and even within one language, a single verb may combine with either auxiliary (either depending on the meaning/context, or with no observable semantic motivation, sometimes depending on regional variation of the language). The auxiliary selection criterion therefore also identifies core classes of unaccusative and unergatives (which show the least variation within and across languages) and more peripheral classes (where variation and context effects are observed).
Other tests that have been studied involve passivization (see Impersonal passive voice), ne/en cliticization in Italian and French, and impersonal, participial, and resultative constructions in a wide range of languages. In Japanese, the grammaticality of sentences that appear to violate syntactic rules may signal the presence of an unaccusative verb. Such sentences contain a trace located in the direct object position that helps to satisfy the mutual c-command condition between numeral quantifiers and the noun phrases they modify (Tsujimura, 2007).
Modern English only uses one perfect auxiliary (have), although archaic examples like "He is fallen/come" reflect the use of be with unaccusative verbs in earlier stages of the language.
The identification of unaccusative verbs in English is based on other criteria. For example, many unaccusatives alternate with a corresponding transitive construction where the unaccusative subject appears in direct object position:
Unaccusative past participles can be used as nominal modifiers with active meaning, while unergative past participles cannot:
Finally, unaccusative subjects can generally be modified by a resultative adjunct. This is a property shared by direct objects and passive subjects, but not shared by the subjects of unergative and transitive verbs.