List of Topics [1.14.4+]

Some sample code can be found here.  The project contains simple working examples of the important concepts in Minecraft and Forge.

Forge also provides some official documentation here.

List of Topics

The Client<-->Server division
How Forge starts up your code (@Mod)
Minecraft Distributions (Client vs DedicatedServer)
Blocks
Block Shapes (VoxelShapes)
Model Basics (eg PigModel)
Model Basics - Rotation

Redstone

Networking:

~Client<-->Server communication
    Vanilla Client<-->Server Communication
    Client<-->Server communication using your own custom messages (SimpleNetworkWrapper)
    Thread safety with Network Messages

Containers

Rendering:
  Block Rendering
  Block Models
  Baking Models
  Item Models
  Quad Rendering
  Lighting