Verb - Valency

The number of arguments that a verb takes is called its valency or valence. Verbs can be classified according to their valency:

• Avalent (valency = 0): the verb has neither a subject nor an object. Zero valency does not occur in English; in some languages such as Mandarin Chinese, weather verbs like snow(s) take no subject or object.
• Intransitive (valency = 1, monovalent): the verb only has a subject. For example: "he runs", "it falls".
• Transitive (valency = 2, divalent): the verb has a subject and a direct object. For example: "she eats fish", "we hunt nothing".
• Ditransitive (valency = 3, trivalent): the verb has a subject, a direct object, and an indirect object. For example: "He gives her a flower" or "She gave the watch to John".

A few English verbs, particularly those concerned with financial transactions, take four arguments, as in "Pat₁ sold Chris₂ a lawnmower₃ for $20₄" or "Chris₁ paid Pat₂ $20₃ for a lawnmower₄".

Weather verbs are often impersonal (subjectless, or avalent) in null-subject languages like Spanish, where the verb llueve means "It rains". In English, they require a dummy pronoun, and therefore formally have a valency of 1.

Intransitive and transitive verbs are the most common, but the impersonal and objective verbs are somewhat different from the norm. In the objective the verb takes an object but no subject; the nonreferent subject in some uses may be marked in the verb by an incorporated dummy pronoun similar to that used with the English weather verbs. Impersonal verbs in null subject languages take neither subject nor object, as is true of other verbs, but again the verb may show incorporated dummy pronouns despite the lack of subject and object phrases.

English verbs are often flexible with regard to valency. A transitive verb can often drop its object and become intransitive; or an intransitive verb can take an object and become transitive. For example, the verb move has no grammatical object in he moves (though in this case, the subject itself may be an implied object, also expressible explicitly as in he moves himself); but in he moves the car, the subject and object are distinct and the verb has a different valency.

In many languages other than English, such valency changes are not possible; the verb must instead be inflected in order to change the valency.