مَرَرْتُ بِمَسَاجِدَ
Translation: I passed by mosques. مَسَاجِدَ is genitive with fatḥa.
مَرَرْتُ بِالْمَسَاجِدِ
Translation: I passed by the mosques. الْمَسَاجِدِ is genitive with kasra.
Adding the definite article al- allows kasra here. The syntactic case remains genitive; only the case marker changes.
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Munṣarif and Ghayr Munṣarif — Definitions, Causes and Parsing
مَرَرْتُ بِمَسَاجِدَ
Translation: I passed by mosques. مَسَاجِدَ is genitive with fatḥa.
مَرَرْتُ بِالْمَسَاجِدِ
Translation: I passed by the mosques. الْمَسَاجِدِ is genitive with kasra.
Adding the definite article al- allows kasra here. The syntactic case remains genitive; only the case marker changes.