Read the sentence given below:

John killed the spider.
Here the noun John is the subject. It is the answer to the question, ‘Who killed the spider?’

The group of words ‘killed the spider’ is the predicate. The predicate contains the verb killed.
What/whom did John kill? – The spider. Spider is the object which John killed. The noun spider is therefore called the object.

When a noun or a pronoun is used as the subject of a verb it is said to be in the nominative case. When a noun or pronoun is used as the object of a verb it is said to be in the objective or accusative case.

Notes
To find the noun or pronoun in the nominative case, put who? or what? before the verb. To find the noun or pronoun in the accusative case, put whom? or what? after the verb.

Note that a noun or pronoun which comes after a preposition is also said to be in the accusative case.

The cat is under the bed.

Here the noun bed is in the accusative case because it is the object of the preposition under.

Note that English nouns have the same form for the nominative and the accusative.

Related posts:

1. Parts of Speech I
2. Noun: Gender
3. Proper Nouns and Common Nouns
4. Collective Nouns and Abstract Nouns
5. Formation of Plural Nouns Part II