I have found that often students draw a blank when the words "nominative case, objective case, or possessive case pronouns" are used. "What?" they often seem to say, think, or express on their faces. It is SO HELPFUL if we teachers give our students a way to remember what all those gobbledygook words mean, and the way I do this is to use the word case as the "trigger word." This, hopefully, will help them to remember the briefcase I use. The briefcase belongs to a man whose initials are N.O.P., which stands for nominative, objective, and possessive, of course.
I also use the words themselves to help students remember when to use them.
Nominative case is used for subjects and predicate nominatives. (And I lean forward and stress the word nominatives.)
Objective case is used for -- surprise -- direct objects, indirect objects, and objects of the preposition. (More leaning forward and stressing the word objects here.)
And, possessive case is used when someone possesses something. At this point of the lesson I grab the briefcase or my coffee cup and loudly say, "I POSSESS this! It is MINE, MINE, MINE!" Then I have them grab something on their desks and say, "I POSSESS this! It is mine!" Yes, they really do it, and they giggle a bit too, and they usually remember it.
It is also helpful to use sentences to help them double-check to see if they have used the correct case.
For nominative case pronouns, the sentence is "_____ went to the store."
Then we all point to each other and say that sentence with all of the nominative case pronouns in order like this.
"I went to the store. You went to the store. He went to the store. She went to the store. It went to the store. We went to the store. They went to the store."
The same process is used for objective case pronouns, but the object is a gift, and the sentence for this one is "The gift is for me. The gift is for you, . . . etc."
For possessive case pronouns, the same chant is used, only we do it twice for each one; first as an adjective and secondly as a pronoun.
"It's my cup; the cup is mine. It's your cup; the cup is yours . . . . "
It is helpful to remember that if the possessive pronoun is functioning as an adjective, there is always a noun behind it.
And, if it is functioning as a pronoun, the PRONOUN stands ALONE (said with emphasis on the long "o" sounds.) 