In النَّفَّاثَاتِ, the article lām assimilates into nūn. The resulting doubled nūn also requires ghunnah. Thus two related features must be recognized: sun-letter assimilation and nasalization on nūn mushaddadah.
By contrast الْعُقَدِ begins with a pronounced article lām because ʿayn is a moon letter. Sharing an initial alif-lām does not mean the same articulation in every word.
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Al-Falaq: doubled nūn and the sun-letter lām
Applied Tajwīd Diagnostics: Four Short Sūrahs
In النَّفَّاثَاتِ, the article lām assimilates into nūn. The resulting doubled nūn also requires ghunnah. Thus two related features must be recognized: sun-letter assimilation and nasalization on nūn mushaddadah.
By contrast الْعُقَدِ begins with a pronounced article lām because ʿayn is a moon letter. Sharing an initial alif-lām does not mean the same articulation in every word.