Hi in this you're making some mistakes.
Don't use opacity for hide the submenus, set it with the property
display:none
Set with
position:relative
your li and the ul inside them withposition:absolute
View this demo an make any question http://jsfiddle.net/pQhpu/214/
EDIT
To resolve your request of centering the submenus in relation with his parent you can use Jquery. I created this function for you: Review the demo here http://jsfiddle.net/pQhpu/264/
$(document).ready (function () {
$('.menu li').mouseenter(function (){
var $this = $(this),
$sub =$(this).children('ul'),
pad = parseInt($this.css('padding-left'),10)+parseInt($this.css('padding-right'),10),
move=($this.width()+pad-$sub.width())/2;
$sub.css ('left',move+'px');
});
})
All you have to change here is the name route of your li that displays the submenu; in my case is '.menu li'
. This function takes the width of the submenu and his parent and make an operation to make it centered.