فَإِنَّهُ مَبْنِيٌّ بِالْفِعْلِ عَلَى السُّكُونِ وَمُعْرَبٌ بِالْقُوَّةِ.
Translation: Such a word is actually fixed on sukūn when cited alone, but potentially declinable: within syntax it can receive grammatical case.
Bi-l-fiʿl refers to its present citation form; bi-l-quwwa to its grammatical potential. This is not permanent inherent indeclinability.
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The indeclinable noun: definition, rule and eight classes
فَإِنَّهُ مَبْنِيٌّ بِالْفِعْلِ عَلَى السُّكُونِ وَمُعْرَبٌ بِالْقُوَّةِ.
Translation: Such a word is actually fixed on sukūn when cited alone, but potentially declinable: within syntax it can receive grammatical case.
Bi-l-fiʿl refers to its present citation form; bi-l-quwwa to its grammatical potential. This is not permanent inherent indeclinability.