While each of the three modes for the Landscape tools allows you to interact with the Landscape in a completely different manner, the keyboard and mouse keys that you use to interact with them are very similar. Here is a rundown of some of the most common keys, key combinations, and mouse buttons that are used when trying to interact with the Landscape tool.
Will allow you to select Landscape components.
Left Mouse Button
Heightens or increases the heightmap or selected layer's weight. For example, in Sculpting mode, this will raise the Landscape heighmap. In Paint mode this will apply the selected material to the Landscape.
Shift + Left Mouse Button
Lowers or decreases the heightmap or selected layer's weight. For example, in Sculpting mode, this will lower the Landscape heightmap. In Paint mode, this will erase the selected material that was applied to a particular section of the Landscape.
Ctrl + Z
Undoes last action.
Ctrl + Y
Redoes last undone action.