مَا مَفْتُوحٌ الْبَابُ الْآنَ.
Mā maftūḥun al-bābu al-āna shows reliance on negation; al-bābu remains nominative. Mā here is a negative particle, not a case governor of the participle.
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After Negation
Ism al-Mafʿūl: Formation, Weak Roots and Government
مَا مَفْتُوحٌ الْبَابُ الْآنَ.
Mā maftūḥun al-bābu al-āna shows reliance on negation; al-bābu remains nominative. Mā here is a negative particle, not a case governor of the participle.