Error: applying the explicit-noun rule to every sentence with a dual subject.
Correct distinction: «ضَرَبَ الزَّيْدَانِ» has an explicit subject, so the verb is singular; «الزَّيْدَانِ ضَرَبَا» has a dual subject pronoun attached to the verb, so the verb is dual.
Error: assuming every grammatically feminine noun requires feminine agreement.
Correct: the rule differs for real versus non-real feminine subjects and according to whether separation occurs.
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Rules of the Fāʿil: Explicit Nouns, Pronouns, Number and Gender
Error: applying the explicit-noun rule to every sentence with a dual subject.
Correct distinction: «ضَرَبَ الزَّيْدَانِ» has an explicit subject, so the verb is singular; «الزَّيْدَانِ ضَرَبَا» has a dual subject pronoun attached to the verb, so the verb is dual.
Error: assuming every grammatically feminine noun requires feminine agreement.
Correct: the rule differs for real versus non-real feminine subjects and according to whether separation occurs.