[SDL] confused by SDL_KEYDOWN event!!!

[dh11111]

here is my code:
 void Tank::HandleInput( SDL_Event &event,bool &run)
{ SDL_PollEvent(&event);
 if(event.type==SDL_QUIT)
 {
  run=false;
  std::cout<<"you have pressed X button\n";
 }
 if(event.type==SDL_KEYDOWN)//if you type event.type=SDL_KEYDOWN there will always be true for the state below
 {
  switch(event.key.keysym.sym)
  {
  case SDLK_a:
   dir=LEFT;
   current=surfacebox[0];
   break;
  case SDLK_s:
   dir=DOWN;
   current=surfacebox[1];
   break;
  case SDLK_d:
   dir=RIGHT;
   current=surfacebox[2];
   break;
  case SDLK_w:
   dir=UP;
   current=surfacebox[3];
   break;
  case  SDLK_SPACE:
   manager.AddBullet(this->Shot());
   break;
  default:
   dir=STAY;
   break;  }  Move(); } if(event.type==SDL_KEYUP) {
  if(event.key.keysym.sym != SDLK_a
   &&event.key.keysym.sym != SDLK_s
   &&event.key.keysym.sym != SDLK_d
   &&event.key.keysym.sym != SDLK_w)//ignore any other key-unpressed event
  {
    Move();
  }
  else
  {
    ResetVel();
    Move();
  }
 }}  problem is that when i keep pressing a key such as "a", the tank in my game keeps moving left,but if i move the mouse during the tank's moving,the tank stops moving.In the doc it seems that there are  some differences between  SDL_KEYDOWN  and SDL_MouseMotionEvent,but in my application ,it seems there no differences between them! what's the difference between these two event type? it seems a little confused.PS: My OS is Windows xp,SDL version is 1.2.12,IDE is VS 2005 PRO. 
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