عُثْمَانُ is a proper name with an additional final alif and nūn, a relevant cause in the classical classification.
جَاءَ عُثْمَانُ — ʿUthmān came; nominative subject.
مَرَرْتُ بِعُثْمَانَ — I passed by ʿUthmān; genitive in function with fatḥa.
This previews another cause studied later. One must not infer the cause from the mere presence of any final nūn.
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Definiteness and Proper-Name Status in Diptote Rules
عُثْمَانُ is a proper name with an additional final alif and nūn, a relevant cause in the classical classification.
جَاءَ عُثْمَانُ — ʿUthmān came; nominative subject.
مَرَرْتُ بِعُثْمَانَ — I passed by ʿUthmān; genitive in function with fatḥa.
This previews another cause studied later. One must not infer the cause from the mere presence of any final nūn.