Some intransitive verbs require a word or phrase to complete the predicate and make sense of the sentence. Such verbs are called verbs of incomplete predication. Examples are: is, am, are, was, were, become, look, seem, appear, taste, smell, grow, turn etc.

The earth is round.
Honey tastes sweet.
The milk turned sour.

The word or words required to make the sense complete is called the complement of the verb. In the above sentence, the words round, sweet and sour are the complements of the verbs is, tastes and turned respectively.

When the complement of a verb says something about the subject, it is called a subject complement.

Read the following sentence.

Harry is a doctor.

Here the noun a doctor is the complement of the verb is. As it says something about the subject Harry, it is called a subject complement.

More examples are given below.

She is beautiful. (Subject complement – beautiful)
This house is to let. (Subject complement – to let)
The baby continued crying. (Subject complement – crying)
She looked upset. (Subject complement – upset)

Object complements

Transitive verbs have objects, but some transitive verbs require, besides their objects, some complements to complete their meaning.

The teacher appointed John monitor.
They elected Peter their president.
Frustration drove him mad.
The parents named her Mary.

Here the complements monitor, their president, mad and Mary say something about the objects John, Peter, him and her. A complement which says something about the object is called an object complement.