A verb is a part of speech that usually denotes action ("bring", "read"), occurrence ("to decompose" (itself), "to glitter"), or a state of being ("exist", "live", "soak", "stand"). Depending on the language, a verb may vary in form according to many factors, possibly including its tense, aspect, mood and voice. It may also agree with the person, gender, and/or number of some of its arguments (what we usually call subject, object, etc.).
The number of arguments that a verb takes is called its valency, or valence. According to valency, a verb can be classified as one of:
- Intransitive (valency = 1): the verb only has a subject. For example: "he runs", "it falls".
- Transitive (valency = 2): the verb has a subject and a direct object. For example: "she eats fish", "we hunt rabbits".
- Ditransitive (valency = 3): the verb has a subject, a direct object and an indirect or secondary object. For example: "I gave her a book", "She sent flowers to me".
It's possible to have verbs with valency = 0. A few of these appear in Spanish and other languages and may be termed "impersonal verbs". For example: Llueve = "It rains". Such verbs don't exist in English because in this language every verb must have a subject, even if it's a dummy one like "it".
English verbs are often flexible with regards to valency. A transitive verb can often drop its object and become intransitive; or an intransitive verb can be added an object and become transitive. Compare:
- I gave. (intransitive)
- I gave blood. (transitive)
- I gave blood for John. (ditransitive)
In the first example, the verb give describes the idea of giving, in the abstract; in the second, what was given is specified; in the third, both the gift and the recipient are set forth.
In many languages other than English, such valency changes aren't possible like this; the verb must instead be inflected for voice in order to change the valency.
A copula is a special kind of verb, sometimes called a linking verb, that is used to describe its subject or equate or liken the subject with its predicate. Because copulas do not describe actions being performed, they are usually analysed outside the transitive/intransitive distinction. The most basic copula in English is to be; there are others (like, seem, become, etc.).
Some languages (the Semitic family, Russian, Chinese and others) can omit the simple copula equivalent of "to be", especially in the present tense. In these languages a noun and adjective pair (or two nouns) can constitute a complete sentence. This construction is called zero copula.
Verbal noun and verbal adjective
Most languages have a number of verbal nouns that describe the action of the verb. In Indo-European languages, there are several kinds of verbal nouns, including gerunds, infinitives, and supines. English has gerunds, such as seeing, and infinitives such as to see; they both can function as nouns; seeing is believing is roughly equivalent in meaning with to see is to believe. These terms are sometimes applied to verbal nouns of non-Indo-European languages.
In the Indo-European languages, verbal adjectives are generally called participles. English has an active participle, also called a present participle; and a passive participle, also called a past participle. The active participle of give is giving, and the passive participle is given. The active participle describes nouns that are wont to do the action given in the verb, e.g. a giving person. The passive participle describes nouns that have been the subject of the action of the verb, e.g. given money. Other languages apply tense and aspect to participles, and possess a larger number of them with more distinct shades of meaning.
In languages where the verb is inflected, it often agrees with its primary argument (what we tend to call the subject) in person, number and/or gender. English only shows distinctive agreement in the third person singular, present tense form of verbs (which is marked by adding "-s"); the rest of the persons are not distinguished in the verb.
Spanish inflects verbs for tense/mood/aspect and they agree in person and number (but not gender) with the subject. Japanese, in turn, inflects verbs for many more categories, but shows absolutely no agreement with the subject. Basque, Georgian, and some other languages, have polypersonal agreement: the verb agrees with the subject, the direct object and even the secondary object if present.