Two questions guide the ordering of definite subject and predicate:
1. Which person or description is already the established topic for this hearer?
2. Which relation or identity does the hearer need to learn about that topic?
Example: If a hearer knows he has a brother but does not know the brother’s name, «أَخُوكَ زَيْدٌ» is apt. If the hearer knows Zayd but does not know that he is his brother, «زَيْدٌ أَخُوكَ» fits better.
This is not merely the marking of syntactic case. It is a choice governed by the communicative circumstances: the topic presents what is being discussed, while the predicate supplies what is new about it.