Compare:
سَكْرَانُ → سَكْرَىٰ: the feminine is not فَعْلَانَةٌ, so this cause applies.
نَدْمَانٌ → نَدْمَانَةٌ: the feminine is on فَعْلَانَةٌ, so this adjective remains declinable through this test.
The decision depends on the feminine form, not merely the visible ending ـان.
Another qualifying pair is عَطْشَانُ → عَطْشَىٰ; its feminine is likewise not on فَعْلَانَةٌ.
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Cause of Ghayr Munsarif: Additional Alif and Nun
Compare:
سَكْرَانُ → سَكْرَىٰ: the feminine is not فَعْلَانَةٌ, so this cause applies.
نَدْمَانٌ → نَدْمَانَةٌ: the feminine is on فَعْلَانَةٌ, so this adjective remains declinable through this test.
The decision depends on the feminine form, not merely the visible ending ـان.
Another qualifying pair is عَطْشَانُ → عَطْشَىٰ; its feminine is likewise not on فَعْلَانَةٌ.