ثُمَّ كَلَّا سَيَعْلَمُونَ
Sentence structure
ثُمَّ connects this renewed warning to the preceding one with a sense of sequence and heightened emphasis. كَلَّا سَيَعْلَمُونَ repeats the deterrence and warning.
ثُمَّ — then, moreover
Grammar: A conjunction that normally indicates sequence with an interval; here it also intensifies the repeated warning.
كَلَّا — No indeed!
Grammar: A particle of deterrence and rebuke, coordinated with the earlier كَلَّا.
سَيَعْلَمُونَ — they will soon know
Grammar: سَ marks future time. يَعْلَمُونَ is a third-person masculine plural imperfect verb and one of the five verbs. “و” is wāw al-jamāʿah: an attached subject pronoun. The final “نَ” is the indicative nūn; its retention marks the verb as indicative.