وَفُتِحَتِ ٱلسَّمَآءُ فَكَانَتْ أَبْوَٰبًا
Sentence structure
Two passive-event descriptions are joined. ٱلسَّمَآءُ is the passive subject of فُتِحَتْ, then كَانَتْ takes an understood pronoun referring to the sky and أَبْوَٰبًا as its predicate.
وَفُتِحَتِ — and the sky will be opened
Grammar: وَ is a conjunction. فُتِحَتْ is a third-person feminine singular perfect passive verb; the final kasrah facilitates pronunciation before ٱلـ.
ٱلسَّمَآءُ — the sky
Grammar: A definite nominative feminine noun and the passive subject نائب فاعل of فُتِحَتْ.
فَكَانَتْ — then it will become
Grammar: فَـ links the result. كَانَتْ is a third-person feminine singular perfect deficient verb; its noun is the understood هِيَ.
أَبْوَٰبًۭا — gateways
Grammar: An indefinite accusative broken plural and the predicate of كَانَتْ. Its singular is بَاب.