In linguistics, an unaccusative verb is an intransitive verb whose grammatical subject is not a semantic agent. In other words, the subject does not actively initiate, or is not actively responsible for, the action expressed by 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 to that 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.[1] The term "unaccusative verb" was first used in a 1978 paper by David M. Perlmutter of the University of California, San Diego.[2] Perlmutter credited the linguist Geoffrey K. Pullum with inventing the terms "unaccusative" and "unergative".[3]