Forge is used to craft metal tools, weapons and furnitures.
How to build a forge
Ingredient:
If you want to craft with forge, you need to build the forge under a roof and an enclosed area.
How to build a forge 
Edit
It can be unlocked when you acquire copper. You need to a workbench near in order to place it down.
Ingredient: Edit
- Select Hammer that in your inventory
- Press [F] to switch to [Crafting] tab
- Press [Q/E] to switch to forge
- Press [LMB] to place down
