couple of theories:
1.
The spanish story: Fake
The modern story: Real
The future story: Fake
But it's not as simple as that. The spanish story is just a story Izzi is writing, so she better can understand her death. Tommy is the main character in the story because he is the one reading it.
There isn't so much to talk about the modern story. Tommy is just searching for a cure against cancer and Izzi is just trying to live her life.
Now... The future story. This is where it really gets exciting. The bubble is actually his subconsciousness. A little copy of Tommy running around and meeting all the things he thinks of. Suddenly he meets Izzi and then it's Isabel from the spanish story. It's all fragments from his memory that comes to attention. Thats why the tree dies at the exact moment Izzi dies. He wants to save Izzi, so he places the tree in the dying star. Accepting death. When he then dies by the star, he starts on a new life and lets Izzi go.
2.
The past is izzy book, the Modern time is his past, the future is the present time, in the begining of the movie is izzy book when the father is about to swing his sword , tom from the bubble wakes up. Hes also doing jitsu which people believe this is an excercise to live a longer life , he is struggling to finish the book that izzy wrote thas why he goes back to the modern time to his last moments with her, he then realizes death is the cure , he dies then hes in heaven with izzy last scene is shot in white, he says i finished it, she says is everything alright? He says yes.
3.
Spanish = The book Izzy is writing
Present = Now
Future = Tom writing the book
Remember Izzy asking him to finish it. It is a way for him to deal with her death. Nothing more... nothing less. The movie is a love story. And Tom writes the ultimate love story... with the search for eternal life, so that he can save her.
4.
my mind just got F'd