مُوَحَّدَةً means the noun must be singular, not dual or plural.
أَبُوكَ: your father; singular, so this condition is satisfied.
أَبَوَاكَ: your two parents; dual, following its own case rules.
آبَاءٌ: fathers; plural, not the singular six-noun pattern.
The meaning “father” or “brother” is insufficient: examine the word form as well.
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Noun Case Classes: Type 4 — The Six Nouns
مُوَحَّدَةً means the noun must be singular, not dual or plural.
أَبُوكَ: your father; singular, so this condition is satisfied.
أَبَوَاكَ: your two parents; dual, following its own case rules.
آبَاءٌ: fathers; plural, not the singular six-noun pattern.
The meaning “father” or “brother” is insufficient: examine the word form as well.