جَاءَ عُمَرُ - nominative with dammah.
رَأَيْتُ عُمَرَ - accusative with fathah.
مَرَرْتُ بِعُمَرَ - without ال or idafah, genitive with fathah.
This is the familiar ghayr munsarif case pattern; the new task is recognizing why عُمَرُ belongs to it.
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Case Application with Umar
Cause of Ghayr Munsarif: Transformation (Adl)
جَاءَ عُمَرُ - nominative with dammah.
رَأَيْتُ عُمَرَ - accusative with fathah.
مَرَرْتُ بِعُمَرَ - without ال or idafah, genitive with fathah.
This is the familiar ghayr munsarif case pattern; the new task is recognizing why عُمَرُ belongs to it.