I don't understand what you mean by "train technique using a tree"? Do you mean hit the tree? Poor tree...
It's a bit complicated and lengthy to go into any detail to do any justice. It's far easier to show and let you feel it in person.
Just quickly, mental will is yi
- intent. Various mental visualization exercises are used to strengthen this.
Qi is something else altogether. If you just stand in a relaxed manner, shoulder width apart, arms by your side. Sink your weight, stand like the top of your head is suspended from the ceiling by an invisible cord, bend your knees slightly, and imagine your tail bone is being pulled into the ground by another invisible cord. "Sit" like you're about to sit in an invisible chair. Breathe normally.
After a while, you will start to notice your arms lift and fall slightly as you breathe in and out. That's the qi. Normally, this is weak, obviously.... since we don't normally move this way, it's entirely foreign. The idea is to strengthen the feeling of the "pull and grab" on all parts of your body, so that when one part moves, the whole moves. BTW, that's what "breathing thru the skin" means. It feels like your skin is being pulled and grabbed with your breath cycle.
Now compare this to the opening sequence of the Yang-shi taiji form - "lift hands". Most people simply lift their arms and hands - usually initiated from the shoulder joint. Instead, try lifting your hands in the same way as when you were standing above. Now, try and catch the feeling of doing this, throughout the entire form!
It is quite possible, of course, to do a form using what most people would consider "normal" and relaxed movement. That's fine too. But it goes back to what I was talking about - unless you know and understand what it is you are training, chances are, you're not training it. You might *think* you are, but it's no different to normal "external" movement, even though you're doing it in a "relaxed" manner. You'd be better off working out at the gym or dancing or something... seriously...
How to use against a resisting opponent? That's jumping the gun a little, but I'll try.
Firstly, it's stupid to struggle against someone who is resisting - what happens is it becomes a contest of strength. Obviously the stronger person will win. My philosophy is, if they want to win so badly, I let them. Ever heard of the classical phrase "invest in loss"? That's what it means. This is the hardest thing for the alphas and betas (i.e. alpha wannabees) to comprehend....
The best way to explain is by using an example.
You know "tug-o-war"? If you both pull against each other, that's resisting. However, if you suddenly let go, what happens? If the rope were a rod and you're both pushing against each other, that's resistance. What if you suddenly turned and let go of the rod? What happens? Ideally, you want to find the path of least resistance, and not give your opponent any opportunity to resist.
BTW, bear in mind that all of this is only a very small part of the "bigger picture". I would suggest you talk to your teacher and ask these questions of him/her. If he/she knows anything (and you will know if they do, or if they're just giving you a load of BS), it is quicker for them to show you.
Once you feel
the power, I guarantee you, you will never want to go back to "external".