وَإِذَا ٱلسَّمَآءُ كُشِطَتْ
Sentence structure
The sky is nominative in the coordinated condition, and كُشِطَتْ is the passive perfect explaining its being stripped away.
Ayah summary
The sky will be stripped open and what lay beyond it will become visible.
وَإِذَا — And when
Grammar: وَ coordinates the whole conditional construction with the preceding one. إِذَا is a future non-jussive temporal adverb containing conditional meaning.
ٱلسَّمَآءُ — the sky
Grammar: The noun is nominative; here it is the deputy subject of an omitted passive verb explained by كُشِطَتْ.
كُشِطَتْ — is stripped away
Grammar: A feminine singular perfect passive verb. The written verbal clause is explanatory and has no syntactic position: the sky is stripped away.