Additional teaching example — not from the book:
بُنِيَتْ مَسَاجِدُ — Mosques were built; «مَسَاجِدُ» is a nominative passive subject.
رَأَيْتُ مَسَاجِدَ — I saw mosques; «مَسَاجِدَ» is an accusative object.
مَرَرْتُ بِمَسَاجِدَ — I passed by mosques; «مَسَاجِدَ» is genitive with fatḥa.
The broken plural «مَسَاجِدُ» follows ṣīghat muntahā al-jumūʿ, another cause of diptote status.