رَجُلٌ جَاءَنِي
A man came to me.
«Rajulun» is an indefinite subject; «jāʾanī» is a verbal predicate including a concealed subject and an object suffix. The sentence illustrates the interaction of an indefinite topic, its grammatical licensing, and the event asserted about it.
This construction is not automatically exclusive. Often the important information is simply that an otherwise unidentified man came, with the predicate clause specifying the event.