A ghayr munsarif noun follows two basic signs when it stands without ال and without idafah:
- it does not take tanwin;
- in the genitive, it normally takes fathah instead of kasrah.
Thus إِبْرَاهِيمُ is nominative, while both accusative and basic genitive are إِبْرَاهِيمَ.
This is a rule about an established noun class. A word does not become ghayr munsarif merely because someone removes its tanwin.