Jack woke up in the jungle in Episode 1. He also said that he didn't remember the crash.
That's because he wasn't in the crash. In episodes "316" and "the life and death of jeremy bentham", one plane survivor of the recent crash said that the oceanic 6 people just "disappeared". We know that Jack wound up "disappearing" to the very spot he was found in the Oceanic 815 crash.
So while he was on the oceanic 815 plane, he transported down to the island just like he did in the recent crash.
I am not sure what this means - does it mean that he had been to the island before, possibly as a young child?