«Lan» negates a future-oriented imperfect verb and governs the subjunctive: «لن يحضرَ زيدٌ», “Zayd will not attend.” The exact extent and strength of the denial depend on the context.
One must not infer everlasting impossibility from «lan» in every context. Here the significant rhetorical feature is its future-directed denial, which differs from other negatives.