Rule 3 states that an additional long-vowel letter is replaced by hamzah when it occurs after the alif in the فَعَائِلُ pattern. Both the extra nature of the lengthening letter and the specified plural structure matter.
صَحِيفَةٌ → صَحَائِفُ
Translation: Written sheet / document → sheets / documents.
This is a pattern-based substitution. Not every long vowel in Arabic turns into hamzah.
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Rule 3: A Lengthening Letter Becomes Hamzah
Miscellaneous Morphological Explanations: Eight Rules for Wāw, Yāʾ, Alif and Hamzah
Rule 3 states that an additional long-vowel letter is replaced by hamzah when it occurs after the alif in the فَعَائِلُ pattern. Both the extra nature of the lengthening letter and the specified plural structure matter.
صَحِيفَةٌ → صَحَائِفُ
Translation: Written sheet / document → sheets / documents.
This is a pattern-based substitution. Not every long vowel in Arabic turns into hamzah.