Corners in real-life aint even a perfect 90o angle, if you look at any corner you will see that it's a bit rounded. This is one of the ways to do it for Half-Life 2.
A more complex setup with objects that have physics, is having them constrained to an angle, perhaps a swingdoor, or a board hanging in a rope for example.
You will learn how to make three kinds of glass here. Unbreakable, breakable like in Half-Life, and shattering, like in Half-Life 2. This can be used for anything breakable.
You see ropes lying around everywhere inside of the game. Valve made a couple of special entities to make them without any hassle, you can make awsome rope fast
Basicly it's just a sliding door but it has a piece of breakable glass kind of attached to it. They are bound together so they will move like they are one piece.
Theese kinds of doors are the ones you see at supermarkets and stuff, they will slide into the wall, or up into the ceiling if you want them to. Very easy to setup.
To make your levels even a bit attractive you WILL need some texturing skills. This video will hopefully introduce you to using the texture dialog box tools and settings.
In this video you will hopefully learn the very basics of what the tools to your left are used for. Some of them will be explained in other video tutorials a bit later on.
Hammer World Editor comes with an excellent terrain painter tool. With it you can easily adjust and create realistic looking terrain, such as hills and mountains.