Hey heinekenbottle, I think you done a typing error, shouldnt it be as follows? Note the '+=' instead of just '='.
Code:
while(1)
   {
      my.x += 5 * (key_z - key_y) * time_step;
      my.z += 5 * (key_i - key_u) * time_step;
      wait(1);
   }

Anyway, another way to look at it is like so (and a bit more flexible IMHO).
Code:
function move_up() 
{ 
   //Sorry I cant read this comment so I'll leave up & down unchanged
   wizard.z += 5; // addiere jedes mal, wenn diese Funktion aufgerufen wird 5 Quants auf die Höhe von wizard 
} 

function move_down()
{
   wizard.z -= 5;
}

function move_ahead()
{   
   wizard.x += 5*time_step*; 
}

function move_back()
{
   wizard.x += -5* time_step; 
}


function main()
{
   //video_mode = 7; // 800x600 pixels
   video_switch (8,32,1);
   level_load ("Drohne_Levelc.wmb");
   //   
   while(1)
   {
      if(key_u == 1)  move_up();
      if(key_j == 1)  move_down();
      if(key_z == 1)  move_ahead();
      if(key_y == 1)  move_back();
      wait(1);
   }
}


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