ʿImrān: proper name + additional alif-nūn = diptote.
Saʿdān: common plant noun; no proper-name status = fully declinable.
Sakrān: adjective with feminine sakrā = diptote.
Nadmān (companion): adjective with feminine nadmāna = fully declinable.
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A Side-by-Side Comparison
Diptote Causes — Additional Alif and Nūn and Their Conditions
ʿImrān: proper name + additional alif-nūn = diptote.
Saʿdān: common plant noun; no proper-name status = fully declinable.
Sakrān: adjective with feminine sakrā = diptote.
Nadmān (companion): adjective with feminine nadmāna = fully declinable.