As long as I've read Mona and Alison were on the -A team but in the 图书 there is the twins story and Alison and Courtney change places so the girls were never 老友记 with Alison they were friend with Courtney and she was the one who died and she was in a mental hospital.
The first A was Mona Vanderwaal. She found Ali/Courtney's Diary in the huge pile of things Maya put on the sidewalk, so she took it and learned the Liar's secrets, and then stalked them to figure out the rest.
She accidentally used her real phone to text Hanna an A message, and when the Liars found out she tried to convince Spencer to become another A but she refused, so she attacked her and in self defense Spencer killed her.
Then Alison came back and started stalking the girls and became the new A. Her parents eventually introduced her to the world as Courtney and she befriended the girls and then locked them in a house and tried to burn it down. The girls escaped but Coutney was trapped. Her body was never found but she was assumed dead.
