Tonfa - there's no conjugation for number in japanese. They add a marker of quantity to the sentence instead. Aaaaanyway... How do you know your karate works? Well, if you test it against other people in the course of training then you know it works in a simulation of combat. Maybe this simulation isn't the same as real fighting, but neither is a car crash simulation in the factory and we still trust our lives to the data they produce. If you aren't satisfies with how close a simulation your class provides, then maybe try finding a closer simulation: MMA or a kickboxing match or become a nightclub bouncer! I'm happy with the simulation myself!