Copulas and Complements
Still another kind of verb, called a copula, though it does not take a direct object, makes only part of the assertion. The rest is made by means of another word (or element) which characterizes the subject but which is an essential part of the assertion. In fact, the complement, as the element completing the verb is called, is the most significant part of the predicate; the verb is merely a bit of machinery by means of which the complement is turned into an assertion.
The sky is red.
The dark shape was a house.
The car looks cheap.
He is in the house.
Home is where the heart is.
In these sentences, red, a house, cheap, in the house, and where the heart is, are parts of the predicate, though they characterize the subject. Such elements are called complements because they complete the assertion of the verb; if adjectives, they are called adjective complements, or better, predicated adjectives; if nouns, they are called noun complements, or predicated nouns; if clauses or phrases, they are called complements, or predicated phrases or clauses. The verbs by means of which these complements are predicated are called copulas.