Twelve Letters [ Explanation of the Ending, post credit scene & some questions ]

 AdD:

So can anyone help me understand, since ATang's mother disappeared when she was small, then how exactly the body remain after so many years, what could be inside the sack, I know for sure its her mom, but still didn't fully understand.

This is a valid question tbh. In the original timeline, Wu fengying was murdered by ye yibo when A'tang was small, as stated by ye yibo Himself in the last episode. Ye yibo had hidden her dead body somewhere in shiqiao for years, until he heard a news in television in 1991 that the area of shiqiao will be renovated soon then he started to panic and asked someone named 'Li' to help him dispose the body offering him payment. They successfully disposed the body inside the sack in foothills before Tang yixun and A'tang arrived there a bit late, as older ye haitang in the car 2026 told older ye yibo "i was one step late to arrive back then,letting you live these extra years "...Actually it doesn’t make sense that the body would be inside the sack intact for 5/6 years, logically dead body Should be decomposed after several years and it should be just her remains bones in There. But considering how a psychopath ye yibo really is,i wouldn’t be surprised if he kept her dead body frozen and intact for years somewhere in shiqiao to use it Against her daughter someday. 

After Ye haitang arrived a bit late in foothills in 1991 Original timeline,she was enraged to find out her father was the one who killed her mother and she also found out about Li and him disposing her mother's body just before her arrival, so she and Tang yixun got engaged in a fight with them where Li got killed and ye yibo ran off and disappeared ever since.Tang yixun took the blame for murder of Li and ye haitang accused ye yibo of murdering wu fengying to police , but as no dead body was found by the police so they didn’t convict or list ye yibo as criminal.