Now apply the book's rule to a fresh example. These sentences are instructional additions, not quotations from the book.
حَضَرَ طَالِبٌ. — A student arrived. طَالِبٌ is a nominative subject.
رَأَيْتُ طَالِبًا. — I saw a student. طَالِبًا is an accusative direct object.
سَلَّمْتُ عَلَى طَالِبٍ. — I greeted a student. طَالِبٍ is genitive after the preposition عَلَى.
The noun stays the same in identity; the changing governor and syntactic role change its final case sign.