جاءني رجل
A man came to me.
رجل جاءني
A man came to me (with ‘a man’ made prominent).
In the first construction «رجل» follows the verb as its grammatical subject. In the second it is fronted and the following verbal clause can be the predicate. In an appropriate context this order may correct a misconception about kind or number.
But making an indefinite noun the mubtadaʾ is itself governed by grammatical conditions. Neither its permissibility nor its restrictive effect can simply be taken for granted.