Intransitive verbs do not have objects:

The baby cried.
She slept.

In the example sentences given above the intransitive verbs cried and slept have no objects, but the sentences still make complete sense.

Now consider the examples given below.

The earth is …
Alice seems …
The milk turned …
Honey tastes …

You will have noticed that the sentences given above do not make complete sense because their predicates (is, seems, turned, tastes) are incomplete. Each of these sentences requires a word or words to make the sense complete.

Now read the sentences given below:

The earth is round.
Alice seems upset.
The milk turned sour.
Honey tastes sweet.

When the adjective round is added to the fragment ‘The earth is’, the sentence becomes meaningful. Similarly the adjective upset turns the fragment ‘Alice seems’ into a meaningful sentence.

Such a verb which requires a word or words to complete the predicate is called a verb of incomplete predication. Examples are: be (is/am/are/were/was), appear, become, look, seem, smell, grow, turn etc.
The word or words required to make the sense complete is called the complement of the verb.

The complements of intransitive verbs can be nouns, pronouns, adjectives, adverbs, participles, prepositional phrases and infinitives.

Alice is my sister. (Here the complement is the noun sister.)
This bag is mine. (Here the complement is the pronoun mine.)
He is nice. (Here the complement is the adjective nice.)
The child fell asleep. (Adverb)
This machine is of no use. (Prepositional phrase)
This house is to let. (Infinitive)
She continued reading. (Present participle)
She seemed distressed. (Past participle)

Here the complements sister, mine, nice, asleep, to let etc., say something about the subjects Alice, bag, he, child, house etc. They are therefore called subject complements.

Object complements

Transitive verbs have objects. But some transitive verbs require, besides their objects, some complements to complete their predicates:

Examples are given below:

The teacher appointed John monitor.
The people elected her President.
Success made him mad.
The court declared the man guilty.
We found him dishonest.

Here the complements monitor, president, mad, guilty and dishonest say something about the objects John, her, him, the man etc. They are therefore called object complements.