A great doubt

Some Jedis become spirits when them dead, other not. Why? It only will be explain in Episode III? Sorry my english, isn't very good.

 

Yep, just wait for Episode III. I think they're planning to explain how it works. But at this point, my personal theory is that they can just come back as ghosts only if they really need to. To help and guide others, or to reconcile with past deeds. And Sith probably don't have force ghosts at all, because they are consumed by the Dark Side when they die. Or something like that. We'll see.

BTW, Welcome to the forum!

 

Your english is godly in comparison to the fact that this question has been asked FAR TOO LONG. You're new, though, so don't feel bad

Short, quick answer: Lucas has promised to answer this plot thread in Episode III.

BUT... just for the hell of it, here's my own theory(I'm spoiler-free for Episode III): it depends on how strong the Jedi are with the force. Obi-Wan and Yoda were clearly both very strong with the force, and they vanished after dying(Though, some argue that Obi-Wan disappears BEFORE Vader's sabre strikes him down). From a certain point of view, Qui-Gon Jinn was the Han Solo of TPM--the rebel who defied conventional wisdom. He's the one who tries convincing an apathetic Jedi Counsil into training the young and dangerous Anakin Skywalker. He's not "in touch" with the Force as others. That's why his body doesn't disappear and become one with the force.

 

I disagree that Qui-Gon isn't in thouch with the Force, in fact I bet he could have prevented Anakin from turning (I also believe that Obi-Wan & Yoda realised how flawed the old Jedi order was).

 

^^^ agreed, infact I think the only reason Yoda and Obi-Wan figure out how to do it is because Qui-Gonkept some of his identity in death, his voice to Anakin/Yoda.