Do not immediately label اثْنَيْنِ as genitive just because you see the -ayn form: the same form can be accusative.
رَأَيْتُ اثْنَيْنِ. — accusative direct object.
مَرَرْتُ بِاثْنَيْنِ. — genitive noun.
Rule: Look at the governor and the word’s syntactic function, not only the visible yāʾ.
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Noun Case Classes: Type 5 — The Dual and Related Forms
Do not immediately label اثْنَيْنِ as genitive just because you see the -ayn form: the same form can be accusative.
رَأَيْتُ اثْنَيْنِ. — accusative direct object.
مَرَرْتُ بِاثْنَيْنِ. — genitive noun.
Rule: Look at the governor and the word’s syntactic function, not only the visible yāʾ.