The underlying representation reconstructs the shape before the specific change, while the surface form is the form actually used. A reason is meaningful only if those two are distinguished.
مِفْتَاحٌ → مَفَاتِيحُ
Translation: One key → keys.
The analytical issue is the position and vowel environment of the alif in the derived plural, not merely the visible presence of yāʾ.
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Underlying Form Versus Attested Form
Miscellaneous Morphological Explanations: Eight Rules for Wāw, Yāʾ, Alif and Hamzah
The underlying representation reconstructs the shape before the specific change, while the surface form is the form actually used. A reason is meaningful only if those two are distinguished.
مِفْتَاحٌ → مَفَاتِيحُ
Translation: One key → keys.
The analytical issue is the position and vowel environment of the alif in the derived plural, not merely the visible presence of yāʾ.