I dont get the ending

I think the movie never implied that the fit of asthma was supposed to be fatal. Or, more significantly, I don't think he travelled EXACTLY 20 years back -- or it would be night time when he arrived at the past. The most fitting explanation is that he travelled to the time period after Yu saw Lun kiss sky but before she left school.

She did remember him. You read her expression wrong -- it was a little bit of surprise and a little bit of happiness.

The picture shown meant that Lun stayed in the past and went to school with Yu (taught by his own dad, perhaps? hmm) It also kind of implied that the movie took side with the "history can be altered by time travel" camp, though the exact details are a bit sketchy..

If you notice Xiao Yu's impression, you'll realize that Lun actually time travelled back to the moment when Xiao Yu has not found out the secret of time travelling. Maybe the one who played the song can control when he/she wanted to travel to, which makes sense that Xiao Yu is able to travel back in time whenever she wanted to.

let say, think of a date while you play the song, make sense right?

read the sentence of the "Secret" general:The road back hidden in the fast Lane melody
I think Lun perform the piano faster than Yu,So he can back the moment when Xiao Yu has not found out the "Secret".
The "Secret" who plaied fast that can travel early.

here is one explanation that I think is amongst the most logical ones you can come up with.

The time travel music and piano only opens a 20-year back and forth time crack, and the time of 1979 and 1999 is in strict sync,which means, if someone time travels from 1979 Jan 18, he can only get to 1999 Jan 18, exactly same time. Which explains why Xiao Yu is so sad when she heard the building is going to be destroyed on the day of graduation. If she could play into any time she likes,she could always return to the time before the building's gone and meet Lun.

There is a theory about two parallel universes existing when people time travel. And this film followed that theory up to 99%. When Lun asks to get into Xiao yu's house for the first time, he got rejected cause her mother said she's sick and resting. There is a POV shot from the upstairs window behind the curtain, which suggersts that someone is watching Lun. Who could that be? It must be the mid-aged Xiaoyu. Because at that time 20 years back, Xiao Yu's not dead yet, thus the present time Xiaoyu exists. Then in the end, when she dies in 1979, Lun of present time gets into her house, the music is still playing but Xiao yu disappeared. It is because the 1979 Xiao Yu's dead, and the present Xiao yu can no longer exist. In case of her mother's confusing behaviour, I don't think its her mental condition. It is because of the two universes , when something new happens 20 years ago, which changes present time, and changes the mother's memory simultaneously.

The bug of this theory is that if the time of 1979 and 1999 has to be preciously in sync, then how could Lun play back to the time before Xiaoyu finds the secret in 1979? He should get back on the night of the 1979's graduation day, at which time Xiao yu's already dead. I heard rumour says Jay Chou wanted a tragic ending, but the editor didn't agree, and the film is ended with Lun saving Xiaoyu .

But we can still try to explain the theory. Let us suppose the time crack repeats itself endlessly, and each run is from a certain time prior to xiaoyu's dicovery of the secret, say it starts on 1979 Jan 1, and ends on 1979 graduation day mid night. So when Lun played the music at the end of the film, he hits the last key when it turns mid night. As the result, he is ported to the begining of another round of time travel and arrives at 1979 Jan 1. At that time Xiaoyu hasn't found out about the secret, and doesn't know Lun at all. In case of why everyone could see lun and not just Xiaoyu, I think it is because the 1999 building is destroyed and he is stuck in 1979.

The last question is why he travels from 1999night time to 1979 day time if the time has to be in sync? Let's say the time crack starts at 1979 some day's day time. Or he just got knocked out by the steel ball and wakes up a few hours later...haha

I know this theory is not perfect, but I think it's the best you can find to justify the time travel mess in this film.

Okay, here is my attempt to explain the story. Personally, I think there are few inconsistencies in the movie.

(a) The time traveling speed is fixed - 20 years.
- It make more sense. Unless Xiao Yu is a god-send pianist, there has to be some time or day differences. Also, doesn't make sense that Lun can plays to the year 1979.

(b) Lun's visit to Xiao Yu's room - the past Xiao Yu.
- We saw how there could have been someone looking out the window in Lun's unsuccessful first visit. Probably Xiao Yu from the past visiting her future hallucinating mother.
- In the second visit(the room), the music was played, but nobody is there. Obviously, her mother saw Xiao Yu. This means that Xiao Yu came to visit her future mother again. But when Lun came inside, she was hiding like before. It would explained why the tables and chairs were cover blankets. No one really live there.

(b) No body died or had died.
- For the photo of Lun's father and Xiao Yu, Lun's father didn't mention her death. Only said that the photo was their last time together. It is like how you would never visit your high school teacher you hate. This is true especially after he told the story to other students. Xiao Yu went to her graduation ceremony, only because she wanted to see Lun again -- thus the bracelet scene.

(c) The asthma scene, just unconscious.
- Why not? Broken hearted, asthma, not enough oxygen to the brain, fell unconscious, woke up later, and the last scene.
- Lun played the piano, got hurt, fell asleep, woke up in the morning, and walked to her class. It is morning after the graduation, she was the only one there studying by herself.

(d) The last picture.
- Either few days after the ceremony or later on the same day when they met again.
- Lun's father is not there in the last photo -- thus he never see her again.
- I believed that Lun just sneaked in at the photo. Possible that he was invisible to others.

(e) The untold elope scene, and never to return.
- Since everyone think she is crazy, they elope to other places.
- Therefore, Xiao Yu's mother never see Lun, and never believe her daughter until much later. Also, that is why her mother became some what crazy, and doesn't have any picture of the older Xiao Yu.
- I am extrapolating a lot here.

(f) The "I can't explain"
- The white out scene. They are at two different time lines. Just look at the background of the room. It would make sense if they both are in the present. Also, no whiteout can last 20 years. By the way, the whiteout could have been from the future, since she obviously took the piano piece back to her time.

(g) Possible
- Lun died in the construction.
- Xiao Yu died the morning later.
- Too many alternative endings, so they mesh it up together.

"Follow the notes upon a journey, At first sight marks ones destiny. Once the journey comes to an end, return lies within hasty keys"....
Read it very carefully and think about the narrative in the movie. Think about which lines apply to which person. Look at the verses seperately, forget about the details of the time travel and the death, they do not supply answers to the stories meanings.

Me and my sister watched the ending, and it did get a bit confusing. Our interpretation of it was that they both died. Jay was not able to hit the last key in time when the building was torn down, and the girl died of asthma.

The last scene was their meeting in the afterlife :)

Alright, these are my thoughts on the ending...

I think that XiaoYu did die from an asthma attack when she was writing the message on the desk. Hence the "tragic ending" to her story that the father told Jay. When jay found out, he went to the piano building and played the song fast on the piano to travel back. We have already established that he can play the piano uncannily fast (see the piano battle) so he uses this talent to play it faster than usual which is why he goes back to before she dies.
The piano is destroyed therefore he's stuck in the past permanently.

Judging by the girl's expression when she sees Jay, he must have gone back to even before she'd discovered the magical sheet music.
This means that no one thinks she's crazy and she can live a happy normal life now, with Jay.
The picure at the end is like a class photo I assume. Which means that they graduated together. It would also imply that everyone can see him. Which might be because he went to the past -not the future- (so the "1st person you see" rule doesn't apply?) or maybe it's just because the piano was destroyed.

With regards to her mother in the future. I think for the first while of the movie (before she died in the past) XiaoYu would have still been alive and would probably still be living there with her mother because everyone thought she was crazy and she let them think that.
When Jay goes to visit her the 2nd timne, and is let in, The mother says that she's in her room. I think perhaps the mother is not "all there". She might be a little bit crazy herself. At that point, Xiao Yu is dead, and has been dead for 20 years. (Notice the only pictures of her in the house are from 20 years ago). So her dying in the past cancelled out her ever having lived/existed in the present.

I don't know if any of that makes sense... but it's only my interpretation of the film.
I loved this movie because it's one that makes you continue to think for hours after the credits have finished rolling. Everyone is entitled to their own interpretation. It is truly a beautiful movie.