In بِزِينَةٍ الْكَوَاكِبِ, the kasratayn produce an audible sākin nūn. Once the following hamzat al-waṣl is dropped, that nūn encounters the following sākin.
The tanwīn nūn is therefore pronounced with kasrah in continuous reading. This is a phonetic junction rule, not a change to the written root of the first word.
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Tanwīn example: bi-zīnatin il-kawākib
Meeting of Two Sākin Letters — ʿAlā Ḥaddih, Ghayr Ḥaddih and Tanwīn
In بِزِينَةٍ الْكَوَاكِبِ, the kasratayn produce an audible sākin nūn. Once the following hamzat al-waṣl is dropped, that nūn encounters the following sākin.
The tanwīn nūn is therefore pronounced with kasrah in continuous reading. This is a phonetic junction rule, not a change to the written root of the first word.