Why was the word "forever" (أبدًا) mentioned in relation to the eternal abode of the people of Paradise in Surat Al-Bayyinah, but not mentioned concerning the eternal abode of the people of Hellfire therein? And what is the interpretation of that?

The word "never" (abada) is mentioned in the Holy Quran, whether in reference to the inhabitants of Paradise or Hellfire, according to a rhetorical rule: If the context is one of detailing recompense, or elaborating on reward, or intensifying punishment, "never" (abada) is mentioned. However, if the context is concise, it is not mentioned.

For example, in Surah An-Nisa, verse 57, "never" (abada) is mentioned with a detailed description of the believers' recompense, whereas in verse 13 of the same surah, it is not mentioned due to the absence of such detail. In Surah Al-Bayyinah, "never" (abada) is not mentioned with the disbelievers because their punishment is not detailed (verse 6), but it is mentioned with the believers to detail their recompense (verse 8). Thus, detailing the recompense amplifies it and extends the mention of "never" (abada).