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In linguistics, a copula (also sometimes called a linking verb, most often in primary education grammar courses) is a word that is used to link the subject of a sentence with a predicate (a subject complement or an adverbial). Though it might not itself express any action or condition, it serves to associate the subject with a predicate that could not stand by itself.

For example, in the sentence "Bob runs", the predicate "run" is already a verb, so it can be used by itself. In the sentences "Bob is old" and "Bob is a fireman", however, the predicates are an adjective and a noun, so the linking verb "is" (a form of be, the usual copula in English) serves to associate them with Bob.

In English, even the normal present tense of verbs is expressed more commonly with the copulative form "Bob is running", [this is not a copula--it is an auxiliary verb.] while other tenses are expressed without it ("Bob ran"), or with the use of other helper verbs ("Bob will run").

English copulas are:

Some languages use copulas differently, so a sentence like "Bob old" would be a complete and grammatical expression. In Chinese the copula for a noun + adjective combination must be an adverb of degree, e.g. "Bobu hen lao", which translates literally as "Bob very old", or "Bobu bu lao" translating as "Bob not old" (but not "Bobu lao" or Bob old). There are still many other languages e.g. Russian or Hungarian where nouns don't have a copula between them (Russian: "ja chelovek" - I'm a man/human -, Hungarian: ö ember" - he is a man/human). The artificial language Lojban has no copula at all, because all words that express a predicate can be used as verbs. The three sentences above would all have the same form in Lojban: "la bob. bajra", "la bob. tolcitno", and "la bob. fagdirpre". Some languages also have more than one copula for different uses; for example Spanish uses the verb "ser" to link a subject to a predicate that specifies an identity or inherent quality ("Bob is old" would be "Bob es viejo"), and the verb "estar" to link predicates that specify a temporary condition ("Bob is here" would be "Bob está aquí"). Italian does it the same way ("Roberto č vecchio"/"Roberto sta qui"), and Hungarian only uses a copula in the latter case with regard to 3rd person (sg/pl) (Róbert van itt"), but not in the 1st example ("Róbert öreg"). This is to relate a subject to a more temporary condition/state taking place in *space* (very often in the sense of Lojban "zvati").

It should be pointed out that the English verb be can also be used as a standalone predicate when it is used to express mere existence. "I am" is a complete sentence meaning "I exist", and in this case "am" is not a copula but a normal, meaningful verb.

See also: E-Prime, a variant of English without the "is" copula.