In the sound masculine plural, final ya is deleted and the preceding kasrah changes to dammah before ونَ.
بَاقِي → بَاقُونَ
قَاضِي → قَاضُونَ
هَادِي → هَادُونَ
دَاعِي → دَاعُونَ
Two operations are required: remove ya, then change the preceding vowel from kasrah to dammah. Merely attaching ونَ would miss the taught transformation.
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Ism Manqus: Case, Dual and Sound Plurals
In the sound masculine plural, final ya is deleted and the preceding kasrah changes to dammah before ونَ.
بَاقِي → بَاقُونَ
قَاضِي → قَاضُونَ
هَادِي → هَادُونَ
دَاعِي → دَاعُونَ
Two operations are required: remove ya, then change the preceding vowel from kasrah to dammah. Merely attaching ونَ would miss the taught transformation.