Valency indicates the number of arguments that are associated with a particular verb in a sentence. Most verbs are at least monovalent; this means they have one argument, which is the subject of a sentence that is performing the action stated by the verb. There are also divalent verbs, which require both a subject and a direct object upon which the action is performed, and trivalent verbs that also need an indirect object that is part of the action. Valency is related to transitivity of verbs, though they are not identical concepts, as the transitivity is based purely on objects and not the subject.
The valency of a verb is determined by how many arguments must be present in a sentence for it to make sense. There are a few words that are avalent, which means they require no arguments, not even a subject. The word "snows," for example, does not truly require a subject and the phrase "it snows" is often used simply for grammatical clarity.
More commonly, however, verbs are at least monovalent, which means they require a single argument for them to make sense. Verbs with this type of valency must have a subject, but they do not need any type of object. Words like "sleep," "dance," and "jump" can all be monovalent since they do not require a target for the action. They do require a subject, however, that is performing the act.
Divalent verbs are those that require both a subject and a direct object upon which the action is being performed. The word "threw," for example, often has this valency in a sentence like "I threw the ball," as it requires both a subject and the object being thrown. A trivalent verb also requires an indirect object that is part of the action, such as the word "give." In the sentence, "I gave my friend a gift," the verb requires the subject and direct object, "a gift," as well as the indirect object that is the target of the action, "my friend." Valency reduction can occur as a word is presented with fewer arguments than may be otherwise required, such as "I give money."
Transitivity is a concept closely related to valency, though they are not identical. The transitivity of a verb is based only on the number of objects required, without consideration of the subject. While the word "threw" in the sentence "I threw the ball," is divalent, it is considered monotransitive, since it requires only one direct object. A monovalent verb is seen as intransitive, and trivalent words are considered ditransitive.