مَرَرْتُ بِالْمَسَاجِدِ — I passed by the mosques.
بِـ: preposition.
الْمَسَاجِدِ: genitive noun with visible kasra because it has the definite article al-.
A diptote of this pattern takes genitive kasra in this context; the underlying plural pattern does not change.
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Diptote Causes — Ultimate Broken-Plural Patterns
مَرَرْتُ بِالْمَسَاجِدِ — I passed by the mosques.
بِـ: preposition.
الْمَسَاجِدِ: genitive noun with visible kasra because it has the definite article al-.
A diptote of this pattern takes genitive kasra in this context; the underlying plural pattern does not change.