What constitutes an elementary command is pretty much up to the author (and the reader). Hopefully what I describe here will be most of what you need to know to get you started and comfortable.
With most GUIs (Graphical User Interfaces), you are really only considering a single directory at a time. With most UNICES, you can consider an arbitrary number of directories at one time. And for the most part, this is also true of operating system like MS-DOS.
Now what do I mean by ``elementary commands''? Finding out where you are in the directory tree, how to change our current (active) directory, listing files in a directory, copying files, renaming files, deleting files, what is meant by a file, looking at an entire file, looking at the first few lines of a file, looking at the last few lines of a file, who is logged in, what is happening on this computer, editing files (vi and family), editing files (pico), editing files (emacs), turning off or rebooting your system.