You can not post or reply at bthe official devforums of ROBLOX. Only developers who are enough popular can do so. Once you create a good game and you get known, ROBLOX will contact you saying that you now have post permissions at devforums.
As a summary, you can't post at devforums.
Yeah, it sucks.
If you make a part have the material "Glass" and you change the transparency to 0.5 (For example) and you place another part with a surfacegui and then an imagelabel and you try to see the imagelabel through the glass, you won't see it.