Grammar rules are always changing, and acceptance is broader now of usage that never used to be approved.
However, if we want others to be aware of our knowledge or that we at least care about our language, proper cases with nouns and pronouns are quite important.
The three cases are nominative (doer of action), objective (receiver), and possessive (ownership).
English nouns are pretty simple because their nominative and objective cases are the same. And there are three straight-forward rules for noun possessives: 1) if singular, add ‘s; if plural ending without an s (children), add ‘s; if plural ending in an s (boys), add just an apostrophe.
It’s those pesky pronouns that give us trouble as to proper case. Remember that the doer of action asks for a pronoun that fits: __ hit the ball. This would be a subject of a sentence or the subject’s equal (He is the boss or The boss is he).
A pronoun as the receiver of action will fit: The ball hit __. Receivers also follow prepositions (I went with her).
These tips provide one way to gauge pronoun case, but the greatest number of errors are made when two pronouns or a noun and pronoun are used together: "Bob and he plan to go" or "The winners were he and Ray." The first “he” is a doer of action, and the second “he” is equal to the subject “winners.” Therefore both should be “he” instead "him."
The best way to check for the correct pronoun is to say it without the noun or pronoun that is not in question. It is not likely one would say, "him plans to go." And if the pronoun is equal to the subject, just flip it around: bet you wouldn’t say, "him is the winner."
Sometimes a pronoun is followed by a noun that further explains it: "We guys were late, and the boss fired us brothers." Just say it without “guys” or “brothers,” and your ear will say, yes! ("We were late, and the boss fired us").
Just keep in mind the doer ("Hank and she left early") and the receiver ("Jim picked Roy and me"). And if still in question, say it without the noun or pronoun not in question.
Determine the use; then trust your ear.
— Sandi Ekberg taught high school English in Medford for 30 years. If you have grammar questions, email her at firstname.lastname@example.org