Quote Originally Posted by K-chan
I don't get it. So, the memories Cloud was having at the start of the game were all Zack's? What happened to Zack though? 0_o Its been a while since I played the game.
If it's been such a long time since you've played the game, why are you asking this?

Anyway, after Sephiroth was thrown into the mako at the Nibelheim Reactor(roughly 5 years before the beginning of the game), Hojo took Zack and Cloud and started experimenting on them(as shown by Hojo's notes found in the Shinra Mansion). Zack had no reaction to the Jenova cell injections, but Cloud did. On disc 3, go to the basement of the Shinra Mansion to see a flashback that shows this. Anyway, Zack finally helped Cloud escape, and they went off to Midgar. Somewhere along the way, Cloud gets a change of Zack's clothes(I don't remember exactly how). Anyway, while Zack's helping him across the continent, Cloud was still really messed up/sick along the way. Anyway, finally Hojo's goons catch up with them, and Zack's killed, and Cloud takes his sword.
His mind was still so messed up from the Jenova cell injections that his brain couldn't recognize the difference between his own memories and his memories of other people(especially Zack). That's where the confusion started. Eventually, he came to believe that he actually was in SOLDIER. This was aided by his own feelings of inadequacy and guilt.

MistaCloud: they communicated by "writing" on the tubes, I believe.