وَزَائِدَةً عَلَى ثَلَاثَةِ أَحْرُفٍ، أَوْ ثُلَاثِيًّا مُتَحَرِّكَ الْأَوْسَطِ
Translation: And more than three letters, or three letters with a vowelled middle letter.
These are alternative structural conditions: a foreign proper name may have more than three letters, or be a three-letter form with a vowelled middle letter.
Nūḥ is three letters but its middle letter is sākin; therefore, the book treats it as fully declinable.
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Diptote Causes — Foreign Origin and Its Conditions
وَزَائِدَةً عَلَى ثَلَاثَةِ أَحْرُفٍ، أَوْ ثُلَاثِيًّا مُتَحَرِّكَ الْأَوْسَطِ
Translation: And more than three letters, or three letters with a vowelled middle letter.
These are alternative structural conditions: a foreign proper name may have more than three letters, or be a three-letter form with a vowelled middle letter.
Nūḥ is three letters but its middle letter is sākin; therefore, the book treats it as fully declinable.