In يَوْمَئِذٍ تُحَدِّثُ, tanwīn meets tāʾ, so the rule is ikhfāʾ. The same expression occurs again before «يَصْدُرُ» in verse 6.
In يَوْمَئِذٍ يَصْدُرُ, tanwīn meets yāʾ and takes idghām with ghunnah.
The first letter of the following word changes the rule:
... تُحَدِّثُ — tāʾ: ikhfāʾ.
... يَصْدُرُ — yāʾ: idghām with ghunnah.
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Tanwīn before tāʾ or yāʾ
Al-Qāriʿah and Az-Zalzalah — Comparative Tajwīd Practice
In يَوْمَئِذٍ تُحَدِّثُ, tanwīn meets tāʾ, so the rule is ikhfāʾ. The same expression occurs again before «يَصْدُرُ» in verse 6.
In يَوْمَئِذٍ يَصْدُرُ, tanwīn meets yāʾ and takes idghām with ghunnah.
The first letter of the following word changes the rule:
... تُحَدِّثُ — tāʾ: ikhfāʾ.
... يَصْدُرُ — yāʾ: idghām with ghunnah.