عَمْرًا — عَمْرٍو
ʿAmran is accusative, ʿamrin (spelled عمرو with a distinguishing final wāw) is genitive. That graphic wāw is not a plural suffix.
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Accusative and Genitive ʿAmr
The Maṣdar: Meaning, Patterns, Governance and Iḍāfah
عَمْرًا — عَمْرٍو
ʿAmran is accusative, ʿamrin (spelled عمرو with a distinguishing final wāw) is genitive. That graphic wāw is not a plural suffix.