Unaccusative verb

In modern linguistics, an unaccusative verb is an intransitive verb whose grammatical subject is not a semantic agent. In other words, it does not actively initiate, or is not actively responsible for, the action of the verb.
An unaccusative verb's subject is semantically similar to the direct object of a transitive verb or to the subject of a verb in the passive voice.
Examples in English are "the tree fell"; "the window broke". In those sentences, the action (falling, breaking) can be considered as something that happened to the subject, rather than being initiated by it. Semantically, the word "tree" in the sentence "the tree fell" plays a similar role as it does in a transitive sentence, such as "they cut down the tree", or its passive transformation "the tree was cut down".
Unaccusative verbs thus contrast with unergative verbs, such as "run" or "resign", which describe actions voluntarily initiated by the subject. They are called "unaccusative" because although the subject has the semantic role of a patient, it is not assigned accusative case.
In nominative–accusative languages, the accusative case, which marks the direct object of transitive verbs, usually represents the non-volitional argument (often the patient). However, for unaccusative verbs, although the subject is non-volitional, it is not marked by the accusative.
As Perlmutter points out, the same verb such as "slide" can be either unaccusative or unergative, depending on whether the action was involuntary or voluntary.
The term "unaccusative verb" was first used in a 1978 paper by David M. Perlmutter of the University of California, San Diego. According to Perlmutter himself, the terms "unaccusative" and "unergative" were both invented by the linguist Geoffrey K. Pullum.
Tests for unaccusativity.
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; even Early Modern English). 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.
For example, in Dutch and Turkish, unergative verbs can be used in impersonal passive constructions, but unaccusative verbs cannot. In the following example from Dutch, the verb is unergative, describing a voluntary action, and can be made passive:
But a sentence with an unaccusative verb, such as "The concert lasted a long time", cannot be made passive.
In Japanese, the grammaticality of sentences that appear to violate syntactic rules may signal the presence of an unaccusative verb. According to transformational models of grammar, 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).
Unaccusativity in English.
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 therefore 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.
While ""to die" has been classified as an unaccusative verb, like "to fall" and "to arrive"", Dąbrowska (2016) noted that ""to die" is an example of "Unaccusative Mismatch", because "to die"" behaves:
Examples.
Perlmutter (1978) gives examples of various types of unaccusative verbs. He emphasises that the following categories are not definitive, but that alternative classifications are possible.
(a) The verb "be" with adjectives:
(b) Those where the grammatical subject is semantically a Patient: 
(c) Predicates of existing and happening:
(d) Non-voluntary verbs of appearance, sound, smell, etc.:
(e) Aspectual predicates:
(f) Duratives: 
He points out that some verbs can be used in either unaccusative or unergative clauses. If the action is deliberate or willed, the clause is unergative:
Recent developments.
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.