will beware = will be made wise (note the force of the Hiphil). Illustrations: Israel (Exo_14:31); the stubborn (Deu_21:21); Gibeonites (Jos_9:3); Sergius Paulus (Act_13:6-12).
reprove = set right.
and he will understand knowledge = will cause teaching to be discerned. Compare Pro_22:17.