I’m not the expert grammarian that anyone else here is, but I can define those terms. Nouns and pronouns have what is called “case” depending on how they’re used in the sentence. There are three cases in English, and I learned them as Nominative, Genitive, and Objective.
Nominative is used in two ways. First, as the subject of a verb:
Cecil Adams does it again!
I can’t wait to hear what Cecil Adams says next.
Second, as the so-called predicate nominative of a verb like “be”:
Our favorite person is Cecil Adams.
Genitive is easy to spot. It’s used for possession:
We are all under Cecil Adams’ control.
The Objective case is used when the noun is a Direct Object, Indirect Object, or Object of a Preposition:
I give Cecil Adams my undying loyalty.
Great minds think alike; fools question Cecil Adams.
May you live in the shadow of Cecil Adams.
There are a few other fine points, but you get the idea. As you can see by my examples, nouns in general look identical in the Nominative and Objective cases. The reason we have cases at all is because it’s a holdover from Latin, in which nouns don’t, in general, look the same in different cases. Pronouns, however, usually look different in different cases. For example, the pronoun that is “he” in the Nominative is “his” in the Genitive and “him” in the Objective. Try substituting those words for the bold words in the examples above. Similary, for the interrogative pronoun, the three cases are “who”, “whose”, and “whom”. Try substituting those as well.
Trying to replace a pronoun with he/him is good, I guess, but I always prefer to look at what the noun is doing in the sentence. Is it the subject (or predicate nominative) of a verb, or an object?