Application

Fighting
To have skill with taijiquan it is necessary to apply the art successfully against an opponent. In a thorough and convincing manner.

Looks aren't everything
Good choreography and coordination means next to nothing martially. A student may be very good at demonstrating an aesthetically pleasing form yet flounder in combat. Why? Their movements may look fine but they are not adequate for the correct execution of the application.

Beyond form
Applications have a simple role in the syllabus, and a vital one. They teach the student how to move their body in a taijiquan manner relative to a non-cooperative opponent. Form cannot provide this opportunity. Doing a movement accurately in thin air (form) means nothing when faced with a thinking, moving, dangerous attacker.

Bad habits
When a student begins to apply taijiquan movements they always flounder straight away. They rely upon local muscular tension, they fail to move their body appropriately, they force. None of these errors will work against a genuine opponent whom is determined to do you harm. The student must set aside their existing habits of body use and employ only what taijiquan is teaching them.

Unified
The single largest impediment to taijiquan application is the inability to move the body as a unified whole. This means not doing what you normally do. It entails a very different manner of moving. Instead of seeking to move the opponent, you must learn how to move yourself around the attack.

The principle
There is a principle underlying every application. Figure out what the unique principle is... and the application can be adapted and employed in a wide variety of different ways.

Reverse engineering
Train the applications until your brain can dismantle and rebuild them as you see fit. Only then will you truly understand what you have learned.