جَاءَ رَجُلٌ يَسْأَلُ عَنِ الْكِتَابِ.
A man arrived asking about the book.
جَاءَ الرَّجُلُ يَسْأَلُ عَنِ الْكِتَابِ.
The man arrived asking about the book.
The indefinite “rajulun” may introduce a previously unidentified person; the definite “al-rajulu” presupposes some identifiable referent. Definiteness alone does not establish whether the function is anaphoric reference, shared knowledge, magnification, or another rhetorical aim.
If a man was mentioned earlier, “the man” can refer back through anaphoric definiteness. Without that prior context, choosing one specific kind of definiteness as certain would exceed the evidence supplied by the form.