ashley benson

1.“Maybe he fools around with students all of the time. A lot of teachers do. Just ask your dad. -A”
2.“Poor Spencer. Always want Melissa’s boyfriends. But remember, if you halik I tell. -A”
3.“She knew too much. -A”
4.“There’s only so much you can bury, Emily. You’re not done with me yet. – A”
5.“Heads up, BFFs. It’s open season on liars and I’m hunting… -A”
6.“Rat me out, your blood’s next. -A”
7.“Lucky you, Aria! Other girls have to do their homework. You get to do the teacher… -A”
8.“Define ‘desertion.’ Seems like you’re about to lose Emily. Who’s next? -A”
9.“Breaking news, bitches: Spencer Hastings, your own person of interest in my death. -A”
10.“I’m still here, bitches. And I know everything. – A”

First off, the Alison that the liars know is not the real Alison, it is Alisons twin sister, Courtney who is crazy, and is pretending to be Alison. The real Alison is taking Courtneys place at a mental hospital. So, in short - the girls never really knew the real Alison.
fake alison was a jerk to Mona(well she was the most jerkiness) so, after fake alisons funeral, she became A.
mona was getting revenge on the Liars forfake alisons actions and all.
after mona fell off a cliff, instead of spencer, who had found out that mona was A, she was taken to a mental hospital
You may think this was the end of it, but you are wrong.
Real Ali comes back from the mental hospital to visit, then she becomes the segundo A.
after she supposedly gets locked inside a burning house, she Liars think it is all over.... but there is still madami A text messages. did Ali survive, or is there a new A