In linguistics, a copula (plural: copulas or copulae) is a word used to link the subject of a sentence with a predicate (a subject complement). The word copula derives from the Latin noun for a "link" or "tie" that connects two different things.[1] A copula is often a verb or a verb-like word, though this is not universally the case.[2] A verb that is a copula is sometimes called a copulative or copular verb. In English primary education grammar courses, a copula is often called a linking verb. In other languages, copulas show more resemblances to pronouns, as in Classical Chinese and Guarani, or may take the form of suffixes attached to a noun, as in Beja, Ket, and Inuit languages. Most languages have one main copula (although some, like Spanish and Thai, have more than one, and some have none). In the case of English, this is the verb to be. While the term copula is generally used to refer to such principal forms, it may also be used to refer to some other verbs with similar functions, like become, get, feel and seem in English (these may also be called "semi-copulas" or "pseudo-copulas").

The principal use of a copula is to link the subject of a clause to the predicate (grammar). A copular verb is often considered to be part of the predicate, the remainder being called a predicative expression. A simple clause containing a copula is illustrated below: The book is on the table. In this English sentence, the noun phrase the book is the subject, the verb is serves as the copula, and the prepositional phrase on the table is the predicative expression. The whole expression is on the table may (in some theories of grammar) be called a predicate or a verb phrase. The predicative expression accompanying the copula – also known as the complement of the copula – may take any of several possible forms: it may for example be a noun or noun phrase, an adjective or adjective phrase, a prepositional phrase (as above) or another adverb or adverbial phrase expressing time or location. Examples are given below (with the copula in bold and the predicative expression in italics): Mary and John are my friends. The sky was blue. I am taller than most people. The birds and the beasts were there. The three components (subject, copula and predicative expression) do not necessarily appear in that order – their positioning depends on the rules for word order applicable to the language in question. In English (an SVO language) the ordering given is the normal one, although here too certain variation is possible: In many questions and other clauses with subject–auxiliary inversion, the copula moves in front of the subject: Are you happy? In inverse copular constructions (see below) the predicative expression precedes the copula, while the subject follows it: In the room were three men. It is also possible in certain circumstances for one (or even two) of the three components to be absent: In null-subject (pro-drop) languages, the subject may be omitted, as it may from other types of sentence. For example, in Italian, sono stanco means "I am tired", literally "am tired". In non-finite clauses in languages such as English, the subject is often absent, as in the participial phrase being tired or the infinitive phrase to be tired. The same applies to most imperative sentences, such as Be good! For cases where no copula appears, see Zero copula below. Any of the three components may be omitted as a result of various general types of ellipsis. In particular, in English, the predicative expression may be elided in a construction similar to verb phrase ellipsis, as in short sentences like I am; Are they? (where the predicative expression is understood from the previous context). Inverse copular constructions, in which the positions of the predicative expression and the subject are reversed, are found in various languages.[3] These have been the subject of much theoretical analysis, particularly in regard to the difficulty of maintaining, in the case of such sentences, the usual division into a subject noun phrase and a predicate verb phrase. Another issue is verb agreement when both subject and predicative expression are noun phrases (and differ in number or person): in English the copula normally agrees with the preceding phrase, even if it is not logically the subject, as in the cause of the riot is (not are) these pictures of the wall. Compare Italian la causa della rivolta sono ("are", not e "is") queste foto del muro.