جَاءَنِي رَاكِبًا رَجُلٌ
Translation: A man came to me while riding.
جَاءَ: past verb.
نِ: protective nūn.
ي: first-person attached object pronoun.
رَاكِبًا: preposed ḥāl, accusative by fatḥa.
رَجُلٌ: postposed nominative subject; indefinite referent of the ḥāl.
The preposing makes its circumstantial role clear rather than treating it as an ordinary adjective after rajul.