Q. The tai chi form taught to me in my school has no connection to the martial art sparring exercise we do. Why is tai chi often referred to as a martial art?
A. When tai chi form is practiced without the Internal Discipline, it has very little relationship to martial arts. It becomes just a slow, relaxed dance, or, in the other extreme, a poorly executed Karate. Internal discipline teaches one how to mobilize one's powerful internal energy and deliver it efficiently. It is the foundation to fa jing (fa jin) and martial arts applications. Without it, during sparring, one has to use external muscle power not much different from those external martial arts. How one builds this martial art foundation from Internal Discipline in Classical Tai Chi is covered in more detail here:
Martial Arts