Grammatical Case in English

Old English had five cases: nominative, accusative, genitive, dative, and instrumental.

Modern English has three cases:

1. Nominative (also called subjective)
2. Accusative (also called objective)
3. Genitive (also called possessive)

The objective case subsumes the old dative and instrumental cases.

Case refers to the relation that one word has to another in a sentence, i.e., where one word “falls” in relationship to another. The word comes from a Latin word meaning “falling, fall.” In other modern languages, adjectives have case, but in English, case applies only to nouns and pronouns.

Nominative/Subjective Case
When a noun is used as a) the subject of a verb or b) the complement of a being verb, it is said to be in the subjective or nominative case.

The king laughed heartily.
King is a noun in the subjective case because it is the subject of the verb laughed.

The king is the son of Eleanor of Aquitaine.
Son is a noun in the subjective case because it is the complement of the being verb is.

Accusative/Objective Case (This isn't accusing anyone of anything)
When a noun is used as the object of a verb or the object of a preposition, it is said to be in the objective or accusative case.

The king subdued his enemies.
Enemies is a noun in the objective case because it receives the action of the transitive verb subdued; it is the direct object of subdued.

The friends went to a movie.
Movie is a noun in the objective case because it is the object of the preposition to.

Sallie wrote Charlie a letter.
Charlie is a noun in the objective case because it is the indirect object of the verb wrote.

A transitive verb always has a direct object; sometimes, it will have a second object called the “indirect object.” In the old terminology, the indirect object was said to be in the “dative case.” Nowadays, the indirect object, like the direct object, is said to be in the accusative or objective case

Note: Some English teachers may still distinguish (as I once did) between the accusative and the dative, but the most recent college English textbook I have, (copyright 2000), does not even list the term “dative” in its index. As nouns and pronouns in the dative case are spelled the same as those in the objective case, there’s no practical reason to retain the former designation.

Genitive/Possessive Case

Of the three noun cases, only the possessive case is inflected (changes the way it is spelled).

Nouns in the possessive case are inflected by the addition of an apostrophe–with or without adding an “s.”

The boy’s shoe is untied.
Boy’s is a singular noun in the possessive case.

The boys’ shoes are untied.
Boys’ is a plural noun in the possessive case.

This one inflected noun case is the source of error for a great many native English speakers.

English pronouns are also a frequent source of error because they retain inflected forms to show subjective and objective case:

Pronouns in the subjective case: I, he, she, we, they, who
Pronouns in the objective case: me, him, her, us, them, whom

The pronouns you and it have the same form in both subjective and objective case.

Note: Strictly speaking, both my and mine and the other possessive forms are genitive pronoun forms, but students who have been taught that pronouns stand for nouns are spared unnecessary confusion when the teacher reserves the term “possessive pronoun” for words that actually do stand for nouns, like mine and theirs. Like adjectives, my, its, our, etc. stand in front of nouns, so it makes sense to call them “possessive adjectives.”

The objective form whom is almost gone from modern speech; the subjective form who has taken over in the objective case for many speakers. 

From: Daily Writing Tips