Does anyone know how I can start and continue the pure Anna route? I still don't really understand it?
various actions and choices affect her "corruption level", indicated by a small chibi at the bottom right corner of the screen. Angel for pure, devil for slut. Each time one of these appear you will gain 1 stack. if it reaches 10 for whichever each side come first then the end game (or more like first part, given on the upcoming updates) will lock to that route.
to know which route you got, it will happen after the first time you have sex with Anna in the basement (with condom), and after when you see her outfit(s) in the kitchen.
to get to pure route, you need to gain 10 stacks of angel chibi anna. the guaranteed way to do that is to help her clean the floor on monday, and (i think) compliment her in a polite and nice way, instead of sexualizing her. tbh it is easier to gain her corruption than keeping her pure, so best thing to do right now is to keep grinding monday event and not doing anything sexual to her until after the basement event