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A copula is a special type of verb in English, though in other languages other parts of speech may serve in this role. The verb acts as a connector between the subject of the sentence and some sort of modifier. Because of this role, in English, it is often also referred to as a linking verb. The prime example in English, and many other languages, is the verb to be, which most often serves to link the subject of the sentence with the predicate — a part of the sentence that modifies the subject.
Some examples of this use of to be include the following:
In each of these sentences, the verb to be is not being used in the literal verbal sense, meaning "to exist," but it is rather taking the second part of the sentence — the predicate — and using it to modify the first part.
By contrast, in the sentence "I think, therefore I am, " the verb is not a copula, but rather a full verb literally indicating existence. This duality that most linking verbs have can often cause confusion when people attempt to refine their speech to be more grammatically standard.
Most people learn early on that we must use adverbs to modify verbs, and adjectives to modify nouns. This is why, when someone uses the sentence "I ran over here as fast as I could," English teachers or resident grammar mavens may correct the speaker, noting that the word he should use is quickly, since ran is a verb, and fast is an adjective.
The problem comes when people attempt to generalize this rule to copula verbs. The trick is that adverbs should be used to modify action verbs but that does not include all verbs. The verb to feel for example, may be used as an action verb, but may also be used as a copula verb. A common over-correction is in response to the question, "How do you feel?" with the incorrect response, "I feel badly."
In this case, the speaker uses the adverbial form badly rather than the adjectival form bad since he recognizes feel as a verb. In this context, however, feel is used as a copula verb, not an action verb, and so the adjectival form is correct. By using the modifier badly, it is implied that feel is being used as an action verb, with the meaning of the sentence rendered to something like, "My tactile senses function quite poorly." What is meant, of course, is to modify the I with the linking verb, saying something like, "Bad is an apt way to describe the state I am in."
One interesting quirk of copula verbs in Indo-European languages is that they tend to be far more irregular than any other verbs in the language. The verb to be, for example, has eight distinct forms, in contrast to the four or five forms other verbs typically have. Added to the eight forms — be, being, been, is, am, are, was, and were — historically there were an additional four forms — best, art, wast, and wert — giving the verb more than double the forms of regular verbs.