I do, however want to step in and voice my opion o­n CSG Subtract.
It's bad, bad bad bad, don't do it, never do it, we need to remove that button from the tool bar.

It's a habit that I personally would like to see beginners *not* get into the habit of doing.
Using CSG can and may cause *you the mapper* more pain than you may ever know or *realize*.
CSG can and may create unwanted and unneeded splits in your brush work. It can cause more compiler errors than you'll want to try and fix.
The biggest problem with CSG... it's a fast and *easy* way to create openings in walls.

Please take my word (as a so-called Professional mapper) never use it.
Instead, learn to work with your brushes, mold those brushes into the shapes you want.
The Clipper tool is your friend, learn it love it.