Valency

In linguistics, valency refers to the number of arguments that a verb takes. It is the same as the concept of arity in mathematics and computer science. The valency of a clause is said to be that of its verb.
 * 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 arguments of a transitive verb are called the subject and object. The subject is often but not always the agent, the participant carrying out an action on the object.

A verb's valency can change: Luina eats a pear is transitive, but Luina eats is intransitive. Some languages mark verbs for changes in valency; others do not.

Alignment
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 implies the subject; this is called null-subject, or pro-drop when object pronouns can also be dropped. Some languages allow dropping pronouns only when the pronoun 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 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.

Word order
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 noun 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.

Monovalency
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. One such coverb in Mandarin is 在 zai. 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.

Examples: Henrik Theiling's constructed language Tesяfkǝm (pronounced roughly TEHS-aff-kerm) and Pete Bleackley's iljena are monovalent constructed languages. Some linguists believe that strict monovalency is impossible in natural languages, that all languages have predicators with one and two referents. But some languages do have features that lead toward monovalent behavior. In Classical Nahuatl, each verb or noun phrase appears to form a separate clause of sorts.
 * 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.)
 * He said; they heard: (He said to them: in Trique)

Some languages have suppletion for the active and passive voices of certain verbs, such as Greek. It wouldn't be too much of a leap for this to spread throughout a language and lead to a tendency toward intransitive verbs, given the right pressures. Or instead of suppletion, a language could use animacy cues to determine whether the intransitive verb's subject is an agent or patient. This parallels so-called middle voice constructions in English, such as "Milca is baking" (agent) vs. "the cookies are baking" (patient). It also parallels language acquisition in children under five or six, who were seen in one study to rely more on animacy than on word order. Another study found that in transitive sentences, children acquiring English as a first language appear to use some nouns only as subjects and others only as objects.

Verbs and cases
"I suppose it is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail."
 * --Abraham Maslow

"It is also noted that human students tend to overuse the verb pattern 12u3o "undergo" as a quasi-accusative."
 * -- frath:Iljena/Morphology

"There's no need to fear: 'undergo' is here."
 * -- Some stand-up comedian satirizing bad iljena teachers who encourage making sense of a monovalent language by trying to impose familiar European paradigms on it, 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. This sort of grammaticalization is seen in several languages, with "give", "leave", and "arrive" have become adpositions with dative, ablative, and allative meanings. 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, instead expressing possession with an adessive construction: "Do you have the pencil?" literally translates as "Is the pencil near you?".

Unlike known natural languages, the conlang Kēlen has no grammatical verbs. It therefore has 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)

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. Examples from English include be in passive voice or progressive aspect, have in perfect aspect, and do as intensifier and carrier of not, all of which act either as auxiliary verbs or as main verbs depending on context. But there's use of grammaticalization, and then there's overuse.