جَاءَ أَخُوكَ. — أَخُو is nominative with wāw (six nouns).
جَاءَ أَخَوَانِ. — أَخَوَانِ is nominative with alif because it is now dual.
رَأَيْتُ أَخَاكَ. — أَخَا is accusative with alif.
رَأَيْتُ أَخَوَيْنِ. — أَخَوَيْنِ is accusative with yāʾ.
Lesson: Different noun forms built from one root may belong to different case patterns.
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Noun Case Classes: Type 5 — The Dual and Related Forms
جَاءَ أَخُوكَ. — أَخُو is nominative with wāw (six nouns).
جَاءَ أَخَوَانِ. — أَخَوَانِ is nominative with alif because it is now dual.
رَأَيْتُ أَخَاكَ. — أَخَا is accusative with alif.
رَأَيْتُ أَخَوَيْنِ. — أَخَوَيْنِ is accusative with yāʾ.
Lesson: Different noun forms built from one root may belong to different case patterns.