The text's phrase قَائِمٌ مَقَامَ السَّبَبَيْنِ means that this special pattern is treated as having the force of two causes.
The book explains this through plurality and its fixed character, describing the form as though it had been pluralized twice.
This is the traditional grammatical rationale in this text. The practical ending is no tanwīn and genitive fatḥa in the usual indefinite context.
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Diptote Causes — Ultimate Broken-Plural Patterns
The text's phrase قَائِمٌ مَقَامَ السَّبَبَيْنِ means that this special pattern is treated as having the force of two causes.
The book explains this through plurality and its fixed character, describing the form as though it had been pluralized twice.
This is the traditional grammatical rationale in this text. The practical ending is no tanwīn and genitive fatḥa in the usual indefinite context.