مَرَرْتُ بِدَوَابَّ — I passed by animals.
مَرَرْتُ: past tense verb; the attached tāʾ is the subject.
بِـ: preposition.
دَوَابَّ: genitive noun marked by fatḥa because it is a diptote; final بّ has shadda.
The gemination belongs to the word pattern; the final case vowel is placed on that doubled consonant.
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Diptote Causes — Ultimate Broken-Plural Patterns
مَرَرْتُ بِدَوَابَّ — I passed by animals.
مَرَرْتُ: past tense verb; the attached tāʾ is the subject.
بِـ: preposition.
دَوَابَّ: genitive noun marked by fatḥa because it is a diptote; final بّ has shadda.
The gemination belongs to the word pattern; the final case vowel is placed on that doubled consonant.