The principal use of a copula is to link the subject of a clause to the predicate. 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 that 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 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, but here too, certain variation is possible:
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. 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 in which 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. They 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 è "is") queste foto del muro.
The precise definition and scope of the concept of a copula is not necessarily precise in any language. For example, in English though the concept of the copula is most strongly associated with the verb be, there are many other verbs that can be used in a copular sense as well. For example,
- The boy became a man.
- The girl got excited by her new toy.
- The dog grew tired from the activity.
And even more tenuously
- The milk turned sour.
- The food smells good.
- You seem upset.