Why the i suffix on her şeyi?
It is the accusative case. "everything" is a specific direct object (I mean, if you say 'everything' it encompasses a specific group of things, all things). The accusative case is used for specific direct objects :)
I think that would be, mathematically and philosophically, a very interesting thing to claim... Lol!
"Ben her şeyi unuttum."
Translation: I forgot everything.
I have forgotten everything. - Correct.
Why is "I forgot everything" wrong?