214. Is (in its various forms) and several other verbs may be used to frame sentences in which some word or words in the predicate describe or define the subject ([§ 17]).
Such verbs are called copulative (that is, “joining”) verbs.
Is in this use is often called the copula (or “link”).
- Time is money.
- Grant was a tireless worker.
- Macbeth became a tyrant.
- His swans always prove geese.
- The current is sluggish.
- Lions are carnivorous.
- This village looks prosperous.
- The consul’s brow grew stern.
- The queen turned pale.
In the first four examples, the copulative verb (the simple predicate[26]) is followed by a predicate nominative ([§ 88, 2]); in the last five by a predicate adjective ([§ 172, 3]).
The copulative verbs are intransitive, since they take no object. Sometimes, however, they are regarded as a third class distinct both from transitive and intransitive verbs.
215. The verb is is not always a copula. It is sometimes emphatic and has the sense of exist.
- I think. Therefore I am. [That is, I exist.]
- Whatever is, is right. [The second is is the copula.]
Most of the other copulative verbs may be used in some sense in which they cease to be copulative.
- The lawyer proved his case.
- Walnut trees grow slowly.
- Mr. Watson grows peaches.
- The wheel turned slowly on the axle.
- He turned his head and looked at me.