What is the effect of the letter "fa" (فـ) in His (Allah’s) saying, “Then for them is a reward uninterrupted” (falahum ajrun ghayru mamnunin) in Surat At-Tin, compared to His saying, “For them is a reward uninterrupted” (lahum ajrun ghayru mamnunin) in Surat Al-Inshiqaq?

Most scholars believe that the exception in Surat Al-Inshiqaq, in the verse (إِلَّا الَّذِينَ آمَنُوا وَعَمِلُوا الصَّالِحَاتِ لَهُمْ أَجْرٌ غَيْرُ مَمْنُونٍ - Except for those who believed and did righteous deeds, for them is a reward without end), is an exception munqati‘ (discontinuous/disconnected exception), because the exempted is not of the same kind as that from which it is exempted. That is why the letter fa’ (فـ, meaning "then" or "so") is not present.

Whereas the exception in Surat At-Tin is an exception muttasil (continuous/connected exception), which is why the letter fa’ is present in the verse (فلهم - then for them).