كَلَّآ إِنَّهُمْ عَن رَّبِّهِمْ يَوْمَئِذٍۢ لَّمَحْجُوبُونَ
Sentence structure
After deterrent كَلَّآ, attached هُمْ is the noun of إِنَّ. لَّمَحْجُوبُونَ is its emphatic predicate; the two prepositional/time phrases are linked to that predicate.
Ayah summary
On that day, the deniers will be deprived of seeing their Lord.
كَلَّآ — Nay
Grammar: a deterrent particle.
إِنَّهُمْ — Indeed, they
Grammar: إِنَّ is accusative/emphatic; attached هُمْ is its accusative noun.
عَن — from
Grammar: a preposition functioning as the linked complement.
رَّبِّهِمْ — their Lord
Grammar: A genitive noun governed by عَن; attached هِمْ is its genitive possessor. The phrase is linked to مَحْجُوبُونَ.
يَوْمَئِذٍۢ — that Day
Grammar: a time adverb functioning as the linked complement.
لَّمَحْجُوبُونَ — surely will be partitioned
Grammar: لَ is the displaced emphatic lām. مَحْجُوبُونَ is a masculine-plural passive participle and the nominative predicate of إِنَّ, marked by wāw; final nūn is nominal.