Look at the next letter after a nūn sākinah or tanwīn, then classify the sound.
Iẓhār: the six throat letters, as in يَنْعِقُ.
Idghām: the letters of يَرْمَلُون: without ghunnah before lām/rāʾ, with ghunnah before yāʾ/nūn/mīm/wāw, as in مِنْ رَبِّهِمْ and مَنْ يَقُولُ.
Iqlāb: before bāʾ, the nūn sound changes toward mīm.
Ikhfāʾ: before its fifteen designated letters, a concealed nasal transition occurs.
These four classifications concern sākin nūn and tanwīn, not every nūn or mīm in every context.
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Four decisions for nūn sākinah and tanwīn
Comprehensive Revision — Applying Tajwīd Rules
Look at the next letter after a nūn sākinah or tanwīn, then classify the sound.
Iẓhār: the six throat letters, as in يَنْعِقُ.
Idghām: the letters of يَرْمَلُون: without ghunnah before lām/rāʾ, with ghunnah before yāʾ/nūn/mīm/wāw, as in مِنْ رَبِّهِمْ and مَنْ يَقُولُ.
Iqlāb: before bāʾ, the nūn sound changes toward mīm.
Ikhfāʾ: before its fifteen designated letters, a concealed nasal transition occurs.
These four classifications concern sākin nūn and tanwīn, not every nūn or mīm in every context.