Sākin nūn is an n (نْ) without a vowel, as in مِنْ رَبِّهِمْ. Here the next letter is rāʾ, so the rule is determined by rāʾ.
It may occur inside a word or at a word boundary. That distinction becomes especially important for the source’s four one-word exceptions to idghām.
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Recognizing sākin nūn
Sākin Nūn & Tanwīn — Iẓhār, Idghām, Qalb and Ikhfāʾ
Sākin nūn is an n (نْ) without a vowel, as in مِنْ رَبِّهِمْ. Here the next letter is rāʾ, so the rule is determined by rāʾ.
It may occur inside a word or at a word boundary. That distinction becomes especially important for the source’s four one-word exceptions to idghām.