Some verbs require a word or phrase after them to complete their predicate. This word or phrase which completes the predicate of a verb is called its complement. A verb which joins a subject with its complement is called a linking or copular verb. Such a verb is also called a verb of incomplete predication.
The most common copular verbs are be and its forms (is/am/are/was/were), act, become, feel, appear, grow, taste, sound, remain, look, turn, went, ran etc.
The complement that follows a copular verb may be a noun, an adjective, an adverb or an adverbial phrase.
He is an engineer. (Noun)
They are our neighbors. (Noun)
Those are clouds. (Noun)
The food is delicious. (Adjective)
His performance was superb. (Adjective)
The program was entertaining. (Adjective)
God is everywhere. (Adverb)
He was in the next room. (Adverbial phrase)
She remained a spinster. (Noun)
The crowd turned violent. (Violent)
That sounds interesting. (Adjective)
She went abroad. (Adverb)
John ran very fast. (Adverb)
He acted in a treacherous manner. (Adverbial phrase)
Copular verbs do not normally take objects, but sometimes they are used transitively.
She acted well. (Here the copular verb acted merely links the subject she with the complement well.)
She acted her part well. (Here the verb acted has an object (her part). It is, therefore, used transitively.)
I was feeling unwell. (Copulative use because the verb is followed by a complement, and not object)
The doctor felt the patient’s pulse. (Transitive use because the verb felt has an object)
The proposal sounds interesting. (Copulative use because the verb is followed by a complement, and not object)
The general sounded the bugle. (Transitive use because the verb has an object)