dudeqwerty means refer to root as if its an array.

Normal array refereces look like myArray[0]

So if you think of all the instances in _root as array elements, you can refer to them as such:
_root[myClip]._x

And in this case you are evaluating the name of the clip instead of calling it out explicitly, so:

_root["sard"+i]._x = ... //there is no dot between _root and [
if i was equal to 1 for example, this would evaluate to
_root.sard1._x =