A forge, or smithy, is a place a blacksmith works with metal. Forge is also the name for the very hot fire a blacksmith keeps for heating up metal until it is soft enough to work with.
You forget something if you don't do it, but not on purpose. You might forget to do your homework, or go out and forget your keys.
The man in the picture has forgotten to put his pants on before he goes to work.
A forklift, also called a lift truck, fork truck or forklift truck is a fairly small truck that has two prongs on the front that are designed to lift things. Forklifts are commonly used for moving large boxes around inside warehouses.
A form is the shape of something, or a framework made around the edge of an empty space that then is filled to make a shape. The picture is of some forms about to be filled with concrete to make the floor of a building.