A lot of the story is inferred, so you aren't alone here. Another user had expressed
similar questions as to what exactly was happening in the game. The simplest explanation would be you got hit on the back of your head by your wife and ended up suffering memory loss.
The main story, told bluntly, would be something like this:
The man always protected this one girl from bullies, which led to the girl falling in love with him. However, it was one-sided and the man eventually fell in love with someone else. The girl he protected went on a rampage with a knife, killed everyone, and either forced the man to marry her or he married her out of fear (not much difference there). Presumably, the man couldn't reconcile the image of the docile girl he once protected with the bloodstained bride and so he forgot all about the incident and convinced himself that he really, really loved her.
...or something like that xD Your story could be as good as mine. What did you make of it?
Yeah, that particular "stuff changing" mechanic isn't used very much... I think it was used there in that classroom and 1 more time depending on the order in which you access stuff-- I'm not completely sure.
Either way, I'm glad you liked the game, and thanks for taking the time to write a review!