مَرَرْتُ بِمَسَاجِدِ الْمَدِينَةِ — I passed by the city's mosques.
بِـ: preposition.
مَسَاجِدِ: genitive with kasra and the muḍāf.
الْمَدِينَةِ: muḍāf ilayh, genitive with kasra.
The first noun takes genitive kasra here because it enters iḍāfa, unlike bare بِمَسَاجِدَ.
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Diptote Causes — Ultimate Broken-Plural Patterns
مَرَرْتُ بِمَسَاجِدِ الْمَدِينَةِ — I passed by the city's mosques.
بِـ: preposition.
مَسَاجِدِ: genitive with kasra and the muḍāf.
الْمَدِينَةِ: muḍāf ilayh, genitive with kasra.
The first noun takes genitive kasra here because it enters iḍāfa, unlike bare بِمَسَاجِدَ.