رَأَيْتُ رَجُلًا رَاكِبًا
Translation: I saw a man who was riding.
رَأَيْتُ: past verb with attached subject -tu.
رَجُلًا: accusative indefinite object.
رَاكِبًا: in the author’s intended contrast, an accusative adjective agreeing with rajulan in case, number, gender and indefiniteness.
This is the book’s ambiguity warning. An indefinite participle following an indefinite noun may function as an adjective; the fatḥa alone does not prove ḥāl.