The STILL implies that we're talking about the same Summer in both titles, but that doesn't make sense because then it should be titled "I know what you did two summers ago" OR " I still know what you did THAT summer"
Bump. I demand satisfaction
share"I Know What You Did Last Summer" refers to the events of the prologue of the original film.
But now, time has shifted, and so the wording is different. Therefore "I STILL Know What You Did Last Summer" refers to the events of the first film that occurred after the prologue. So he still knows, but now it's a different summer and a different event that he refers to.