بِمَصَابِيحَ — bare indefinite diptote: genitive fatḥa.
بِالْمَصَابِيحِ — with al-: genitive kasra.
بِمَصَابِيحِ الْمَسْجِدِ — in iḍāfa: genitive kasra.
All three are genitive. The ending differs because of the definite article or the construct relationship.
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Diptote Causes — Ultimate Broken-Plural Patterns
بِمَصَابِيحَ — bare indefinite diptote: genitive fatḥa.
بِالْمَصَابِيحِ — with al-: genitive kasra.
بِمَصَابِيحِ الْمَسْجِدِ — in iḍāfa: genitive kasra.
All three are genitive. The ending differs because of the definite article or the construct relationship.