Another deleted files not recovering space thread

Ok I've searched and only found a bunch of dead ends I'm running snow leapord OSX deleting downloaded files emptying trash not gaining any hd space back at all my hd does not grow unless I add to it but does not reduce in size when files are deleted I have an external hd but don't use it for the Mac and don't want to run out and purchase one at this time just want to know how to get the space back because on a pc it would be there as soon as you delete whatever was taking up space and emptied the trash. How do I get the gb back? Please assist... I've also used disk utility to delete free space still no difference.

Something is probably using the space that you either are not aware of or can't see. Download the free "Disk Inventory X" from here and use it to find out what's taking up the space.

The space was taken up by my downloads but as I deleted them and emptied trash the space stayed the same as if they were never deleted. I will get that app and check but am quite confident in what I will find. And simply want to know why you don't recover space when u delete something and how you go about getting it back.