Here is a passage from Cambridge Lessons in English. Book II. Teacher’s Edition, by George Sampson, published 1929. It covers how nouns are sometimes classified according to their role in the sentence and their relationship to other nouns in the sentence:

Nominative and Objective.

A noun or pronoun, subject of a sentence, is in the nominative case.

A noun or pronoun, object of a sentence, is in the objective case. (Some people prefer to call this the Accusative Case. You will use whichever term your teacher prefers.)

Thus, in “She touched him,” She (subject) is in the Nominative Case, and him (object) is in the Objective (or Accusative) Case.

In some languages, Latin, for example, nouns, adjectives and pronouns have different forms for nominative and objective; but in English only the pronouns have this difference of form. Thus we say (i) “John helps Mary” and (ii) “Mary helps John”; but although John (i) and Mary (ii) are both nominative, and John (ii) and Mary (i) are both objective, there is no change of form. In Latin those sentences would be written “Iohannes adiuvat Mariam” and “Maria adiuvat Iohannem.” In English the pronouns change as the nouns do in Latin; and if we put pronouns instead of John and Mary, the sentences would be written thus: “he helps her” and “she helps him.”

The difference between nominative and objective is especially important in the relative pronouns who and whom. Look at these sentences:

Who steals my purse steals trash.

The boy who brought the message was late.

Whom the Lord loveth he chasteneth.

The boy whom he trusted with the message was late.

Why do we say who in (i) and (ii), and whom in (iii) and (iv)?