وَإِنْ كَانَتَا فِي صِفَةٍ، فَشَرْطُهُ أَنْ لَا يَكُونَ مُؤَنَّثُهُ عَلَى فَعْلَانَةَ، كَـ«سَكْرَانَ».
Translation: If the additional alif and nūn occur in an adjective, the condition is that its feminine not follow the pattern faʿlāna, as in sakrān.
For an adjective, the test is the feminine form, not proper-name status.
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Diptote Causes — Additional Alif and Nūn and Their Conditions
وَإِنْ كَانَتَا فِي صِفَةٍ، فَشَرْطُهُ أَنْ لَا يَكُونَ مُؤَنَّثُهُ عَلَى فَعْلَانَةَ، كَـ«سَكْرَانَ».
Translation: If the additional alif and nūn occur in an adjective, the condition is that its feminine not follow the pattern faʿlāna, as in sakrān.
For an adjective, the test is the feminine form, not proper-name status.