إِنَّ رَبَّكَ لَبِٱلْمِرْصَادِ
Sentence structure
إِنَّ opens an emphatic nominal clause. رَبَّكَ is its noun, with attached كَ as possessor; لَبِٱلْمِرْصَادِ is the emphatic prepositional predicate.
Ayah summary
Surely your Lord keeps full watch over everyone's deeds.
إِنَّ — Indeed
Grammar: An accusative/emphatic particle opening a nominal clause.
رَبَّكَ — your Lord
Grammar: رَبَّ is the accusative noun of إِنَّ; attached كَ is a genitive possessor.
لَبِٱلْمِرْصَادِ — (is) surely Ever Watchful
Grammar: لَ emphasizes, بِ is a preposition, ٱل defines مِرْصَادِ; the phrase functions as the predicate of إِنَّ.