وَإِذَا ٱلْبِحَارُ سُجِّرَتْ
Sentence structure
The seas are nominative in the coordinated condition, and سُجِّرَتْ is a Form II passive perfect explaining what happens to them.
Ayah summary
The seas will be set ablaze and their waters will become intensely heated.
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.
ٱلْبِحَارُ — the seas
Grammar: The noun is nominative; here it is the deputy subject of an omitted passive verb explained by سُجِّرَتْ.
سُجِّرَتْ — are filled or set ablaze
Grammar: A Form II feminine singular perfect passive verb. The written verbal clause is explanatory and has no syntactic position.