flash actionscript: 1.how to move the focus of cursor to _root.box13 when _root.box12 is full of
characters;
2.how to move the focus of cursor to _root.box22 when users press "down" arrow
in _root.box12;
without pressing tab.

p.s:where_root.box12,_root.box13,_root.box22 are TextFields

Use the setFocus method:

Selection.setFocus(instanceName);

It has been a while since I looked at this but try something along these lines:

box13.onSetFocus = function(oldFocus){
trace(oldFocus);
}

the onSetFocus method tracks the instance that had focus prior to the setFocus.

I hope that helps,
cheers