In grammar, an intransitive verb is a verb that is associated with only one noun or noun phrase. The number of noun phrases associated with a verb is known in linguistics as its valency, and so an intransitive verb is also called a univalent verb. In English, this one noun is the subject and so an intransitive verb is commonly said to have no object; this distinguishes it from a transitive verb, which has a valency greater than one. Examples of intransitive verbs include to die and to sleep. Transitive verbs include to kill and to buy.
English chooses arbitrarily to mark the one noun associated with an intransitive verb as the subject (I die) rather than as the object (*die me). This convention is known in linguistics as nominative–accusative morphosyntactic alignment.
Other languages, notably Basque, take the opposite convention, marking the single noun associated with an intransitive verb as the object of the sentence rather than its subject. This convention is known as ergative-absolutive alignment.
Many languages mix the two conventions depending on the verb, sometimes due to difference in meaning ("I dance", because I do the dancing, but "die me", because death happens to me) and sometimes for purely habitual reasons. This convention is called split ergativity.
In languages that have a passive voice, a transitive verb can be turned into an intransitive verb by using it the passive voice. For example, consider the following sentence:
In this sentence, "hugged" is a transitive verb taking "Mary" as its object. The sentence can be passivized with the direct object "Mary" as the grammatical subject as follows:
This shift is called promotion of the object.
The passive-voice construction cannot take an object. The passivized sentence could be continued with the agent:
It cannot be continued with a direct object to be taken by "was hugged." For example, it would be ungrammatical to write "Mary was hugged her daughter" in order to show that Mary and her daughter shared a hug.
Intransitive verbs can be passivized in some languages. In English, intransitive verbs can be used in the passive voice when a prepositional phrase is included, as in, "The houses were lived in by millions of people."
Some languages such as Dutch have an impersonal passive voice that allows for the passivization of an intransitive verb that does not have a prepositional phrase. A sentence such as "The children slept" can be passivized in German to remove the subject. The passivization can occur without a prepositional phrase, as in "The children slept in the bed," which, in English, could become "The bed was slept in by the children."
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