A learner may see names such as إِبْرَاهِيمُ and فَاطِمَةُ and conclude that every proper name is ghayr munsarif. That conclusion is incorrect.
Contrast:
- إِبْرَاهِيمُ — ghayr munsarif in the taught example.
- مُحَمَّدٌ، صَالِحٌ، نُوحٌ — fully declinable names; they take tanwin.
Why? Proper-name status alone is not enough. Another recognized cause must combine with it. This is why similar-looking “names” do not all follow one case pattern.