In English, case refers to the form a word takes and its function in a sentence. There are just three cases in the English language:

• Subjective
• Possessive
• Objective

Apostrophy's

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Many indefinite pronouns and "it", "you," and most nouns, have distinctive forms only for the possessive case. Most nouns and indefinite pronouns, that form usually are indicated by an apostrophe: e.g. George's shirt, states' powers; another's duties; someone's car. For "it" the possessive is formed by adding "s"; for "you" the possessive is formed by adding "r" or "rs" to the word.

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CASE

"Who do you think is the culprit?"
"Whom do you think I met in Cape Town?"
"Between you and me ..."
"I knew it was she as soon as I saw her."
No person able to think would make the mistake about the pronoun in any of these examples. As for "whom" at the beginning of a sentence, is very seldom heard of lately.
Never use an apostrophe to form a possessive for "it," "you," or personal pronouns. Except for in answer to "Who's there?" usage sanctions "it's me."
There is little doubt in some minds about this form: "I don't mind your knowing it." But "you knowing it" would quite clearly be wrong. It's the knowing I don't mind, it's not 'you', or your 'knowing.'

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Six personal pronouns have a distinctive form for each of the three cases:
Subjective
"I" "we," "he," "she," "who" and "they" are the forms used for subjects and subject compliments.
Examples of subjects: She and I were good friends. They lived next door. Who teaches English? We grew up together.
Complements of the subject: It is I. Piet Pompies, that's who. The ones responsible are Cooper and Chelsea.

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Possessive, the form used to show ownership is:
"Whose," "their/theirs," "his," "her/hers," "our/ours" and "my/mine," "their/theirs," and "whose."
Examples are:
Before a noun: "My house burnt down," "his dog is vicious," "Our boat is fast," "Who did that,?" "Her hair is wet," "Their dollhouse is beautiful."
Possessors in the noun position:
Whose will be chosen.? His looks skew. Ours is much bigger. Hers was last on the list. Theirs disappeared yesterday.

Objective
"Him," "her," "them," "whom" and "me."
The forms reserved for use as objects of verbs or prepositions are: “Me,” “us,” “him,” “her,” “them” and “whom”
Examples are:
For him this is easy. Sylvester wanted him to leave. Mary was introduced to them. Finding whom I was looking for. Andries drove to the mall to meet us. I returned to my hometown. Percy likes me.

Nouns in English have only one case inflexion: the possessive (or genitive). The case of nouns other than the possessive is regarded as common case. In English, the forms of the subjective (or nominative) case and the objective (or accusative) case are identical. The common case in English grammar is the ordinary base form of a noun, such as dogs, cat, stars, moon, house, car.

" 'Common case' describes nothing and analyze nothing, but grammar is essentially analytical. It names things not for the fun of having a vocabulary but to understand the relations of working parts. One can analyze an English sentence without using the word 'case'; what matters is to know that a given word is subject or object, and of what is the one or the other." (Wilson Follett, Modern American Usage, revised by Erik Wensberg. Hill and Wang, 1998)

Source: https://www.thoughtco.com/common-case-grammar-1689766
Author: Richard Nordquist

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Source: English Grammar: EBH Joubert

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