مَرَرْتُ بِعُمَرَ: ʿUmar is genitive after bi-, marked with fatḥa; cause: ʿadl + proper-name status.
مِنْ أَيَّامٍ أُخَرَ: ukhar is a genitive adjective of ayyāmin, marked with fatḥa; cause: ʿadl + adjectival status.
مَرَرْتُ بِرَجُلٍ أَسْوَدَ: aswad is a genitive adjective of rajulin, marked with fatḥa; cause: original adjectival status + verb-like pattern.
All three visibly end in fatḥa, but the underlying causes differ.
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Causes of Diptote Status — ʿAdl and Waṣf
مَرَرْتُ بِعُمَرَ: ʿUmar is genitive after bi-, marked with fatḥa; cause: ʿadl + proper-name status.
مِنْ أَيَّامٍ أُخَرَ: ukhar is a genitive adjective of ayyāmin, marked with fatḥa; cause: ʿadl + adjectival status.
مَرَرْتُ بِرَجُلٍ أَسْوَدَ: aswad is a genitive adjective of rajulin, marked with fatḥa; cause: original adjectival status + verb-like pattern.
All three visibly end in fatḥa, but the underlying causes differ.