كَلَّا سَيَعْلَمُونَ
Sentence structure
كَلَّا firmly rejects and restrains their attitude. The following verbal clause warns that they will soon know the truth.
كَلَّا — No indeed!
Grammar: A particle of deterrence and rebuke; it rejects the preceding attitude and introduces a warning.
سَيَعْلَمُونَ — they will soon know
Grammar: The attached prefix سَ marks the future. 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.