For a noun: additional alif and nūn + proper-name status; example: ʿImrān.
For an adjective: additional alif and nūn + absence of a feminine form on faʿlāna; example: sakrān.
These are separate tests. Do not treat ʿImrān as an adjective or sakrān as a proper name.
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Diptote Causes — Additional Alif and Nūn and Their Conditions
For a noun: additional alif and nūn + proper-name status; example: ʿImrān.
For an adjective: additional alif and nūn + absence of a feminine form on faʿlāna; example: sakrān.
These are separate tests. Do not treat ʿImrān as an adjective or sakrān as a proper name.