Explanation timelines (SPOILERS)
I read in different threads on the board explanations for the timelines, but I think that the confusion starts at the moment she puts the letter in the mailbox trying to warn him for his death. During the movie it was clear that both were living in the same timeline, but he in the year 1997-1998, she in 1999-2000. All things were clear: scene where he puts the tape recorder in the mailbox that she lost in the past at the station, the wine he reserved for her that she drinks two years later and so on... So let's assume that this was all in timeline 1. It's in this timeline he dies, because she remembered the moment he dies and she wants to warn him by putting a letter in the mailbox. At that moment in the movie, the moment she puts the letter in the mailbox, timeline 2 starts.
For this explanation I use the idea of the what-if question. What if he reads the letter, what if he doesn't read the letter. The what-if creates two possibilities, so two timelines. (For the people who saw Sliding Doors, it's the same idea). So let continue. The movie started in timeline 1 and in this timeline he dies, so he didn't read it, otherwise he was still alive. It makes sense: because if he reads the letter on time, he won't go to see her and he won't die. If she didn’t see him dying, she could never trying to warn him. Otherwise it would create a paradox(*) Because of his death they can't communicate anymore by letters via the mailbox. In this timeline she will never be together with him.
But now the other part. Timeline 2 was created because he reads the letter on time. So he didn't go to help her and he didn't die. All the other events remain the same. Remember that timeline 2 was created the moment she puts the warning letter in the mailbox, so all the events before this split were the same. In timeline 2 he decides to wait to see her at the moment she leaves Il Mare.
Consequence of this meeting is that she won't send the letters to him (from the future to the past). But because he is from the past and standing there, means that he did receive the letters. You can't take away what he already has. But at the moment he meets her, he changes the future (the past cannot be changed). She will not send letters. This has effect that there are no more paradoxes, for example the paradox that she will never send a warning letter to him because she doesn’t see him dying on the street. Because of his intervention of meeting her the moment before she puts the letter in the mailbox, he solves this paradox. Because if the wait at the moment he received the warning letter, you come in the same paradox as described above in (*).
Why meeting her at that moment. I guess it was smart of him. His story is the most believable at that moment, because he can tell what she wrote down on the letter and can also tell that she has the intention for putting that letter in the mailbox.
To summarize:
1) There is a creation of a second timeline the moment she puts the warning letter in the mailbox
2) He changes the future in the second timeline by meeting her when she leaves Il Mare (there is no creation of an other timeline at that moment)
If you had any suggestions/corrections, please let me hear.
Sorry for the type errors. I've tried my best, but English is not my first language.