There are so many commands in AutoCAD and Civil 3D that we use every day. Some of them are available via toolbars, menus, or the ribbon, that you only have to learn where to find them, not memorize them.
In my case, there are a few commands or system variables that just get lost in the deep void of space inside my brain. Here's a few of them:
LINEWORKSHRINKWRAP - Creates a boundary around a selection set of objects.
PELLIPSE - Setting this variable to 1 will draw an ellipse as a polyline.
FILEDIA - This variable gets set to 0 during some commands and/or lisp routines. If AutoCAD crashes, you may have to reset it to 1 to display dialog boxes instead of command line prompts.
'STAE - Transparent command for Civil 3D to select station from the parent alignment of a profile view.
'SSE - Transparent command for Civil 3D to select station from the parent alignment of a profile view and elevation from a surface.
'PSE - Transparent command for Civil 3D to select station and elevation from a profile view.
'ZTP - Transparent command for Civil 3D to zoom to a point or range of points.
There's plenty more to add to this list, but it's a start.