مَسَاجِد: after the alif are ج، د — 2 letters.
دَوَابّ: after the alif is بّ — a doubled letter.
مَصَابِيح: after the alif are ب، يْ، ح — 3 letters, with sākin middle yāʾ.
All three are express examples in the book. Count letters, not vowel marks, and pay attention to the shadda.
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Diptote Causes — Ultimate Broken-Plural Patterns
مَسَاجِد: after the alif are ج، د — 2 letters.
دَوَابّ: after the alif is بّ — a doubled letter.
مَصَابِيح: after the alif are ب، يْ، ح — 3 letters, with sākin middle yāʾ.
All three are express examples in the book. Count letters, not vowel marks, and pay attention to the shadda.