Do not identify «هَذَيْنِ» as automatically accusative: the same form is genitive after a preposition.
رَأَيْتُ هَذَيْنِ: accusative object.
مَرَرْتُ بِهَذَيْنِ: genitive after bi-.
By contrast, «هَذَا» itself remains unchanged; describe its syntactic position.
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Demonstratives — forms, syntax and usage
Do not identify «هَذَيْنِ» as automatically accusative: the same form is genitive after a preposition.
رَأَيْتُ هَذَيْنِ: accusative object.
مَرَرْتُ بِهَذَيْنِ: genitive after bi-.
By contrast, «هَذَا» itself remains unchanged; describe its syntactic position.