| Case refers to the form a word takes and its function in a sentence. The English language has just three cases: subjective, possessive and objective.
Most nouns, many indefinite pronouns and “it” and“you” have distinctive forms only for the possessive case. For most nouns and indefinite pronouns, that form usually is indicated by an apostrophe: John's coat; states' powers; someone's house; another's task. For “it.” the possessive is formed by adding “s”; for “you” the possessive is formed by adding “r” or “rs” to the word.
(Never use an apostrophe to form a possessive for it, you or the personal pronouns noted below.)
Six personal pronouns have a distinctive form for each of the three cases:
“I,” “we,” “he,” “she,”“who” and “they” are the forms used for subjects and subject complements.
Subjects — He and I were great friends. We grew uptogether. They lived next door. Who teaches that course?
Complements of the subject — The ones responsible are Joe and she. It is I. Joe Smith, that's who.
“My/mine,” “our/ours,” “his,” “her/hers,” “their/theirs and “whose” are the formsused to show ownership.
Before noun — My car broke down. Our boat leaks.His dog is ugly. Her back is wet. Their name is Mudd. Whose job is that?
Possessors in the noun position — Mine is green. Ours is over there.His looks heavy. Hers was last inline. Theirs sank yesterday.Whosewill be chosen?
“Me,” “us,” “him,” “her,” “them” and “whom” are the forms reserved for use as objectsof verbs or prepositions.
Sue likes me. Elaine drove to the airport to meet us. For him this is no problem. Sam wanted her to leave. Jim was introduced to them. Finding whom I was looking for, I returned to my favorite pastime.