فَصَبَّ عَلَيْهِمْ رَبُّكَ سَوْطَ عَذَابٍ
Sentence structure
فَصَبَّ is a perfect verb with رَبُّكَ as subject. سَوْطَ is its object and is annexed to genitive عَذَابٍ; عَلَيْهِمْ is linked to the verb.
Ayah summary
So your Lord poured upon them a scourge of punishment.
فَصَبَّ — So poured
Grammar: فَ marks result; صَبَّ is a third-person masculine-singular perfect.
عَلَيْهِمْ — on them
Grammar: عَلَىٰ is a preposition and attached هِمْ a masculine-plural pronoun in the genitive position. This phrase is linked to «صَبَّ».
رَبُّكَ — your Lord
Grammar: رَبُّ is the nominative subject of صَبَّ; attached كَ is a genitive possessor.
سَوْطَ — scourge
Grammar: An indefinite masculine accusative object, annexed to the following noun.
عَذَابٍ — (of) punishment
Grammar: An indefinite masculine genitive noun functioning as the complement of سَوْطَ.