I'm guessing frame is supposed to be the current frame. If that's the case then querying the currentTime command is what you want to do. Basically, anything within the `` will be evaluated so you can get a usuable value that you can assign to a variable or compare to another value or whatever. And to assign your $newPosn value to the cube's translateY, then you want to use the setAttr command.


float $newPosn;
if (`currentTime -q` == 1){
$newPosn = 0.0;
}
setAttr pCube1.translateY $newPosn;
$newPosn = $newPosn + 1.0;