Rule 2 explains that alif becomes yāʾ when preceded by kasra in the stated plural configuration.
مِحْرَابٌ → مَحَارِيبُ
Translation: Prayer niche → prayer niches.
مِفْتَاحٌ → مَفَاتِيحُ
Translation: Key → keys.
The preceding kasra is decisive. The visible yāʾ is a result of morphological substitution and should not simply be counted as a new radical.
Opening interactive controls...
Rule 2: Alif Becomes Yāʾ After Kasra
Miscellaneous Morphological Explanations: Eight Rules for Wāw, Yāʾ, Alif and Hamzah
Rule 2 explains that alif becomes yāʾ when preceded by kasra in the stated plural configuration.
مِحْرَابٌ → مَحَارِيبُ
Translation: Prayer niche → prayer niches.
مِفْتَاحٌ → مَفَاتِيحُ
Translation: Key → keys.
The preceding kasra is decisive. The visible yāʾ is a result of morphological substitution and should not simply be counted as a new radical.