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Predicate Noun or Adjective.
283. With sum
and a few other intransitive or passive verbs, a noun or an adjective
describing or defining the subject may stand in the
predicate. This is called a Predicate Noun or Adjective.
The verb sum is especially common
in this construction, and when so used is called the copula
Other verbs which take a predicate noun or adjective are the so-called
copulative verbs signifying to become, to be made, to be named,
to appear, and the like.