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Examples of intransitive verbs include to age, to die, and to sleep. Transitive verbs include to give.
The valency of a verb is related to transitivity. Where the transitivity of a verb only considers the objects, the valency of a verb considers all the arguments the verb takes, including both the subject of the verb and all of the objects (of which there are none for an intransitive verb).
In languages that have a passive voice, a transitive verb in the active voice becomes intransitive in 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 by 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."
In languages with ergative–absolutive alignment, the passive voice (where the object of a transitive verb becomes the subject of an intransitive verb) does not make sense, because the noun associated with the intransitive verb is marked as the object, not as the subject. Instead, these often have an antipassive voice. In this context, the subject of a transitive verb is promoted to the "object" of the corresponding intransitive verb. In the context of a nominative–accusative language like English, this promotion is nonsensical because intransitive verbs don't take objects, they take subjects, and so the subject of a transitive verb ("I" in I hug him) is also the subject of the intransitive passive construction (I was hugged by him). But in an ergative–absolutive language like Dyirbal, "I" in the transitive I hug him would take the ergative case, but the "I" in I was hugged would take the absolutive, and so by analogy the antipassive construction more closely resembles *was hugged me. Thus in this example, the ergative is promoted to the absolutive, and the agent (i.e. him), which was formerly marked by the absolutive, is deleted to form the antipassive voice (or is marked in a different way, in the same way that in the English passive voice can still be specified as the agent of the action using by him in I was hugged by him—for example, Dyirbal puts the agent in the dative case, and Basque retains the agent in the absolutive).
In many languages, there are "ambitransitive" verbs, which can be either transitive or intransitive. For example, English play is ambitransitive (both intransitive and transitive), since it is grammatical to say His son plays, and it is also grammatical to say His son plays guitar. English is rather flexible with regards to verb valency, and so it has a high number of ambitransitive verbs; other languages are more rigid and require explicit valency changing operations (voice, causative morphology, etc.) to transform a verb from intransitive to transitive or vice versa.
In some ambitransitive verbs, called ergative verbs, the alignment of the syntactic arguments to the semantic roles is exchanged. An example of this is the verb break in English.
In (1), the verb is transitive, and the subject is the agent of the action, i.e. the performer of the action of breaking the cup. In (2), the verb is intransitive and the subject is the patient of the action, i.e. it is the thing affected by the action, not the one that performs it. In fact, the patient is the same in both sentences, and sentence (2) is an example of implicit middle voice. This has also been termed an anticausative.
Other alternating intransitive verbs in English are change and sink.
In the Romance languages, these verbs are often called pseudo-reflexive, because they are signaled in the same way as reflexive verbs, using the clitic particle se. Compare the following (in Spanish):
Sentences (3a) and (3b) show Romance pseudo-reflexive phrases, corresponding to English alternating intransitives. As in The cup broke, they are inherently without an agent; their deep structure does not and can not contain one. The action is not reflexive (as in (4a) and (4b)) because it is not performed by the subject; it just happens to it. Therefore, this is not the same as passive voice, where an intransitive verb phrase appears, but there is an implicit agent (which can be made explicit using a complement phrase):
Other ambitransitive verbs (like eat) are not of the alternating type; the subject is always the agent of the action, and the object is simply optional. A few verbs are of both types at once, like read: compare I read, I read a magazine, and this magazine reads easily.
Especially in some languages, it makes sense to classify intransitive verbs as:
This distinction may in some cases be reflected in the grammar, where for instance different auxiliary verbs may be used for the two categories.
In many languages, including English, some or all intransitive verbs can take cognate objects—objects formed from the same roots as the verbs themselves; for example, the verb sleep is ordinarily intransitive, but one can say, "He slept a troubled sleep", meaning roughly "He slept, and his sleep was troubled."