What are copular verbs?

A copular verb is a special kind of verb used to join an adjective or noun complement to a subject.  Common examples are: be (is, am, are, was, were), appear, seem, look, sound, smell, taste, feel, become and get.
A copular verb expresses either that the subject and its complement denote the same thing or that the subject has the property denoted by its complement.
For example in the sentence ‘Peter is my boyfriend’ the copular verb is asserts that Peter and my boyfriend are the same person whereas in the sentence ‘Peter is British’ the copular verb is assigns the quality of Britishness to Peter.
More examples are given below.

• Honey is sweet. (Here the copular verb is assigns the quality of sweetness to honey.)
• The stew smells good.
• The milk turned sour.
• The night grew dark.
• She became a writer.

After copular verbs we use adjectives, not adverbs.
Compare:

• She spoke intelligently. (Here the adverb intelligently modifies the ordinary verb spoke.)
• She appears intelligent. (NOT She appears intelligently. Appears is a copular verb. It should be followed by an adjective, not an adverb.)

The copular verbs like become, get, grow, go, turn, stay, remain, keep etc., are used to talk about change or the absence of change.

• I am becoming older.
• I am getting older.
• I am growing older.
• The leaves are going yellow.
• The leaves are turning yellow.

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