The grab key was never utilized nor was the use key.
About moving objects. There are two types. There are what are called mover brushes, which are pre-animated by the level designer to move through a specific set of key frames; and there are objects (specifically meshes) which can have their physics set so that the player can move them around (boxes, etc.). Obviously to use mesh objects you need to be able to make, texture and import models. These are the only ones you can have the player move "free-form". Of course you can use the mesh objects provided in the game too.
Aniamted movers are a different matter, as they are actual brushes that are part of the world geometry. They can be animated to move around (not to change size or shape) by assigning them key frames, triggers, and adjusting various properties. They are "added" into the world using the make mover button and commands. You definitely need to read a tutorial to learn how they work.
To learn how the mesh objects work simply load a level that has moveable crates and examine how they were set-up property-wise.
