After Alison (Really it's Courtney) has that fight with Spencer outside the barn where Spencer hits Courtney, Courtney runs and Spencer just remembers seeing her arguing with someone blond.
That's because it's Courtney fighting with Alison. Alison and Courtney fight and it's not specified how she actually killed Courtney but she throws Courtney in the hole behind the barn, the one the next day the workers fill so no one realizes Courtney is in it...
After that, Alison and Courney's parents take Alison away thinking she's Courtney... she doesn't come back till the last book and nobody knows she's the real Alison...
Courtney is alison's twin sister, who pretended to be her
Suffication
Alison
by hands
Yes
Courtney wants to pretend to be Alison meanwhile Courtney was supposed to be in a mental hospital
Yes. Alison had a twin sister named Courtney.
she tried to be alison and she was in a mental ward
yes
Yes
Courtney stole Ali's ring :) hope it helps
Because the Pretty Little Liars weren't actually her friends. Courtney, (Alison's twin sister) made friends with them. Alison had nothing to do with them. So she's mad and tries to kill them.