Both particles make their ism mansub; the surrounding sentence determines which particle is used.
إِنَّ زَيْدًا صَالِحٌ
زَيْدًا is the ism of إِنَّ, so it is mansub with fathah. صَالِحٌ is the marfu khabar.
أَشْهَدُ أَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ
مُحَمَّدًا is the ism of أَنَّ, so it is mansub. The coordinated words عَبْدُهُ وَرَسُولُهُ form the khabar.
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Introduction to Mansubat and the Ism of Inna
Both particles make their ism mansub; the surrounding sentence determines which particle is used.
إِنَّ زَيْدًا صَالِحٌ
زَيْدًا is the ism of إِنَّ, so it is mansub with fathah. صَالِحٌ is the marfu khabar.
أَشْهَدُ أَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ
مُحَمَّدًا is the ism of أَنَّ, so it is mansub. The coordinated words عَبْدُهُ وَرَسُولُهُ form the khabar.