Many languages have noun cases. We don’t teach cases in English, but there definitely are some correlations between cases and English grammar. Maybe it would be a good idea to acknowledge some of the cases used in the students’ native tongue(s) and explain what English uses instead.
As a teacher, you sometimes need to come up with examples on the spot. Sometimes it’s hard to think of a particular example, especially when more common alternatives (ones that don’t fit your category) come to mind far too easily. That’s why it’s good to have some word lists on hand.