I used to train in a martial art but I forget how to perform the techniques
If you ever learned a physical skill at any time in your life, unless you have some mental defect or disease, you will never forget the skill. If you learned to skate at 10 years old and have never skated since then, and you put on skates at 40 years of age, you will be able to skate again in just a few minutes. If you learned a Taekwondo pattern at 10 years old and were proficient at it and then quit training in Taekwondo until you were 40 years old, your should still be able to perform the pattern after just a few minutes of review.
I once studied a martial art
Ever had a new student in class claim to have studied some other martial art, but then he does not know how to do a simple side kick. If you ever learned how to kick properly, you will never forget how to kick properly. The kick may not be as high as before, but it should still be technically perfect.
Your art is too easy for me
Ever had a person say he used to train in a martial art and you ask him to come to your class to train, and he says that your class is too easy for him because his art stressed hard training. The person is not training anywhere now, so how could your class be easier than not training at all.
My techniques are too deadly
Ever had a person says she used to train in a martial art and you ask her to come to your class to train, and she says that her art has such deadly techniques that she would be afraid of harming someone in the class. Yea right! Ask her come to class and show everyone some of her deadly techniques. Assure her that all your students have signed waivers that will not hold her responsible for injuries that may occur.