Search documentation - http://unity3d.com/support/documentation/ScriptReference/Animation.html Detect left mouse button - Input.GetKey("mouse 0") will detect a left-mouse buttom click and replace mouse 0 with left shift for left shift.

So full code:

function Update() {
   if (Input.GetKey("left shift")) { 
      animation.Play("nameOfAnimation", PlayMode.StopAll);
      transform.Translate(transform.forward * (normalSpeed * 2));
   }
}

The animation should be stored in the Animation component of the gameObject