Form without shen is simply a hollow pattern.
You may have the positioning, the movements and the timing right, but that is all.
Knowing the 13 postures will help to develop your sense of jing, but something is still missing.
Performing the applications is not enough either.

Intention is a start, but you need something more. You need spirit. Shen.

People push for things in various aspects of their lives.
If things do not go their way, they push, and then push a little harder.
When learning tai chi, pushing is fruitless because the skills will only come once you stop pushing.

The attitude of not pushing is called wu wei.
For it to really affect your life, it needs to extend beyond the class.

Moving qigong is much easier to practice than form.

The coordination is simpler and the health benefits more immediate and tangible. Form takes much longer to learn and quite a long time to actually perform competently.

Anyone can do qigong. There is a lot less to think about. The exercises are focussed and clear. You need not complicate matters by thinking about self defence applications or any other concerns.

It is also important not to get caught up thinking about qi. Let the energy take care of itself. Keep your mind on the exercise. Intention leads the qi and the qi leads the blood. Breathe and relax.

Once you are competent at qigong, take what you have learned into form. Form is essentially moving qigong with extra features. The extras include increased balance and coordination, martial application and energy emission.

In motion the whole body should be light and agile,
with all parts of the body linked as if threaded together.

The qi should be excited; the shen should be internally gathered.

The postures should be without defect,
without hollows or projections from the proper alignment;
in motion the form should not become disconnected.

The jing should be rooted in the feet, generated from the legs,
controlled by the waist, and manifested through the fingers.

If correct timing and position are not achieved, the body will become disordered
and will not move as an integrated whole; the correction for this defect
must be sought in the legs and waist.

The principle of adjusting the legs and waist applies for moving in all directions;
upward or downward, advancing or withdrawing, left or right.

All movements are motivated by mind, not external form.

If there is up, there is down; when advancing, have regard for withdrawing;
when striking left, pay attention to the right.

If the mind wants to move upward, it must simultaneously have intent downward.

Alternating the force of pulling and pushing severs an opponent's root
so that he can be defeated quickly and certainly.

Insubstantial and substantial should be clearly differentiated.
At any place where there is insubstantiality, there must be substantiality;
Every place has both insubstantiality and substantiality.

The whole body should be threaded together through every joint
without the slightest break.

Tai chi is like a great river rolling on unceasingly.

Wardoff, rollback, push, squeeze, pluck, split, elbow, shoulder are equated to the Eight Trigrams.
The first four are the cardinal directions; the second four are the four corners.

Advance, withdraw, look right, look left and central equilibrium are equated to the five elements: metal, wood, fire, water and earth.
All together these are termed the Thirteen Postures. 

(Chang San-feng)