مَجْرُوْرٌ

A noun moves from its natural nominative state to a genitive state مَجْرُوْرٌ in two instances 1) when it is preceded by a preposition الْمَسْبُوْقُ بِحَرْفِ جَرٍّ or 2) when it is in the possessive case الْمُضَافُ إِلَيْهِ

Genitive case / عَلَامَةُ الْجَرِّ كَسْرَةٌ

Noun / الْاِسْمُ

Primary signs of declension / عَلَامَةُ الْإِعْرَابِ الْأَصْلِيَّةُ