Apply the same diagnostic question to each example: Which letter immediately follows the n-sound?
يَنْعِقُ → ʿayn → iẓhār. مِنْ رَبِّهِمْ → rāʾ → idghām without ghunnah. مَنْ يَقُولُ → yāʾ → idghām with ghunnah. مِنْ بَعْدِ → bāʾ → qalb. تُنْفِقُونَ → fāʾ → ikhfāʾ.
Remember the one-word exception, such as دُنْيَا, before applying the mnemonic mechanically.
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A four-rule comparison
Sākin Nūn & Tanwīn — Iẓhār, Idghām, Qalb and Ikhfāʾ
Apply the same diagnostic question to each example: Which letter immediately follows the n-sound?
يَنْعِقُ → ʿayn → iẓhār. مِنْ رَبِّهِمْ → rāʾ → idghām without ghunnah. مَنْ يَقُولُ → yāʾ → idghām with ghunnah. مِنْ بَعْدِ → bāʾ → qalb. تُنْفِقُونَ → fāʾ → ikhfāʾ.
Remember the one-word exception, such as دُنْيَا, before applying the mnemonic mechanically.