وَإِنْ كَانَ أَحَدُ الِاسْمَيْنِ مَعْرِفَةً وَالْآخَرُ نَكِرَةً، فَاجْعَلِ الْمَعْرِفَةَ مُبْتَدَأً، وَالنَّكِرَةَ خَبَرًا الْبَتَّةَ، كَمَا مَرَّ.
Translation: If one of the two nouns is definite and the other indefinite, treat the definite as mubtadaʾ and the indefinite as khabar, as already explained.
The author states the default analysis of the nominal predication. In «زَيْدٌ مُعَلِّمٌ», Zayd is the definite topic and muʿallim the indefinite predicate; syntactic interpretation must also respect meaning and context.