Complex sentence: A ruler is used to see if a plane has enemy cards in his field of fire: if so, the player chooses one target that must take a damage card with a random amount of points (optional rules cover special damages too).
Split into simpler sentences: A ruler is used to see if a plane has enemy planes (cards) in his field of fire. If so, the player chooses one target that must take a damage card with a random amount of points (optional rules cover special damages too).