you can in certain games, like multiplier, custom games and forge.
Yes, there will be Forge mode in Halo 3: ODST... but on the other hand there is two discs so there might not be in the halo ODST disc.:)
You go on custom games(not forge) and select a map and you're on without forge
Yes, there will be forge mode in Halo 4.
Yes, Halo: Reach will include Forge.
you go on forge and press the up arrow
Not all are free, but you can download forge maps from other people and play them with your friends or alone. You can alose upgrade them.
you can on forge and custom games.
Yes, it will have forge
The map "Forge World" in a Halo: Reach is just a large map with nothing on it, other than like trees and rocks and stuff. There are only 4 items which you can place in Forge mode on "Forge World". 3 of which are in the spawn category.
Most likely in Forge, Custom games and Matchmaking. Campaign? NO
If you mean making your own map, it's Forge mode.