It is easier to say what should not be used taijiquan:
1. NO use of force, no strain or tension whatsoever neither in the muscles nor in the joints.
Tai ji is always a relexed form and internal (nèijiā) martial art that has purpose to restore the natural flow of chi (qi) in the body. The more relaxed your body is, the more power, speed and strenght one has in its movements. It is also one reson why you never see a top boxer with tense arms and torso...
2. Never go bending or breaking the straight line of your central axis.
The line that goes from the center of your head, through your neck and spine and divides the body in two halves must be straight at all times.
One of the tai chi classics states that one never leans on the other person. There is no tilting sidewise and no leaning forwards or backwards.
3. Don't push out the tailbone (weilu) at the lowest of our back - have it tucked under so the back is straight aligned.
When in move have the awareness on the top of our head as well on the legs. The back can be at an angle when pushing or moving forward but it keeps its alignment.
4. Don't lock straight the body, arms and the legs.
Maintain the straightness of the torso, arms and legs, but don't forget the relaxation and flexibility the body needs to adjust when it moves for one to another position.
5. Never put pressure on the legs, especially the knees as they are the most sensitive part. Keep them relaxed even if you have to take a higher stance. For the beginning is good to have higher stance and gradually lower it as the leg muscles and tendons strenghten.
6. Try not to lose the concentration, and have full awareness on the movement. Maintain and keep your practice regular in order to learn the form correclty. Once knowing the right sequence the details can be added and improved.