In linguistics, valency refers to the number of arguments that a verb takes.
- A monovalent or intransitive verb takes one argument: Colin sleeps.
- A divalent or transitive verb takes two arguments: Colin threw the ball.
- A trivalent verb takes three arguments: Bob gave Aerith the ball.
The valency of a clause is said to be that of its verb.
A verb's valency can change: Luina eats a pear is transitive, but Luina eats is intransitive. Some languages mark verbs whose valency changed; others do not.
Languages vary in their morphosyntactic alignment, or what arguments in one valency take the same form as parts of a sentence in other valencies. Languages in which the argument of the intransitive verb looks like a subject, such as English, are called nominative-accusative or just accusative after the argument that appears only in divalent and trivalent expressions. Languages in which it looks like an object, such as Basque, are called ergative-absolutive or just ergative. A few languages, called active or "split-S" languages, act like an ergative language (using the object form) with some intransitive verbs, but they act accusative (using the subject form) with other intransitive verbs. In fact, most ergative languages show traces of this behavior based on the tense, aspect, or person of the verb.
Many languages, such as Spanish and Japanese, drop subjects when either the verb form or context; this is called pro-drop. Some languages allow dropping pronouns only when A. is the subject, B. is definite (that is has been referred to earlier in discourse), or C. both. Otherwise, the sentence must be flipped into a different voice, in which the underlying roles of arguments change. In an accusative language, removing the subject from of a divalent clause requires flipping the sentence into a passive voice, changing the form of the verb and promoting the object to subject: The ball was thrown. Ergative languages flip sentences to antipassive voice when shedding the object.
A few languages have distinct forms for agents, subjects, and objects. Tripartite alignment uses the agent and object forms only with transitive verbs and the subject form with all intransitive verbs. Inuktitut uses agent and subject if the object is definite (the or a proper noun) or subject and object otherwise. In Austronesian alignment, the inflection of the verb dictates which argument takes the subject or "trigger" form, like a more general version of the passive voice system, and speakers use it to place the focus on a particular argument.
Languages also differ in their word order. Some tend toward verb before object (VO), as in English, Italian, Chinese, Arabic, and Welsh. Others tend toward object before verb, such as Japanese, Korean, Hindi, Latin, and any German sentence with a compound verb or subordinate clause. Verbs tend to come on the same side of the verb as case clitics: VO languages tend to have prepositions before their object, and OV languages tend to have the object before the postposition.
In the vast majority of languages, the subject comes before the object. This reflects the psychological tendency to establish a topic before making a comment about it. Some languages, such as Japanese, even reorder the sentence to move the topic to the front instead of using passive voice.
One alignment not yet encountered by real-world scouts is monovalent alignment. A monovalent language has only intransitive verbs. They express the meanings of other languages' divalent and trivalent clauses with serial verb constructions: each argument has a separate verb for each role in an action. Prepositions are considered verbs too, just as in real-world SVC languages such as Chinese and numerous West African languages. Instead of being pro-drop, these languages are clause-drop: an entire noun-verb pair predicted by context can be left out, and utterances may end up very telegraphic once sentences are cut down to one argument.
- Bob gave; ball changed-hands, Aerith received. (Bob gave the ball to Aerith.)
- Bob gave; Aerith received. (Bob gave it to Aerith.)
- Colin threw; ball flew. (Colin threw the ball.)
- Ball flew. (He threw the ball.)
Henrik Theiling's constructed language Tesяfkǝm (pronounced roughly TEH-saff-kerm)  and Pete Bleackley's iljena are monovalent constructed languages. Some linguists believe that strict monovalency is impossible in natural languages; all languages have predicators with one and two referents.
Verbs and cases
"When all you have is a hammer, everything looks like a nail."
- --Abraham Maslow
"It is also noted that human students tend to overuse the verb pattern 12u3o "undergo" as a quasi-accusative."
"There's no need to fear: 'undergo' is here."
- -- Some stand-up comedian satirizing bad iljena teachers who encourage trying to impose familiar European paradigms on a monovalent language, whose students end up speaking what amounts to Engrish.
Not all languages make an absolute distinction between verbs and case-markers such as prepositions. For example, in Chinese, 到 (dào) means both the preposition to and the verb go; arrive. Likewise, Toki Pona tawa means both to and go; leave. And what one language expresses with a verb another may express with some case. For example, a lot of languages such as Russian and Finnish don't have a verb for to have, and they express possession with an adessive construction: "Do you have the pencil?" literally translates as "Is the pencil near you?".
The conlang Kēlen has no grammatical verbs and therefore no concept of "valency" to speak of. When faced with a monovalent language such as iljena, a Kēlen-speaking grammarian might analyze it too as having no verbs but instead a multitude of case transfixes. Some English verbs correspond roughly to cases in this way:
- x is
- ergative (e.g. Gnivad is eating, or an orange is eaten by Gnivad)
- x has
- locative (e.g. Acha has an orange, or an orange is near Acha)
- x receives
- dative (e.g. Staisy received a pear; or, A pear was given to Staisy)
But speakers of another language learning iljena for the first time have been seen to overextend this pattern and use generic verbs for arguments other than what they perceive to be the main one, such as "undergo" for an object. Some grammaticalization of verbs is normal; English be in passive voice or progressive aspect, have in perfect aspect, and "do" as intensifier and carrier of "not" yet retain their identity as verbs. There's use, and then there's overuse.
- Eloi language of the novel The Time Machine by H.G. Wells, hypothesized to be monovalent