Brief Introduction to vim (="visual editor improved") which is a modern Linux version of vi. The main reason why a newbie like myself ever needs vi is for rescue--sometimes it is the only editor available.  The most important thing to understand about vi is a "modal" editor, i.e., it has a few modes of operation between which user must switch. The quick reference is below, the 4 essential commands are in bold.

The commands to switch modes:
The key   Enters the mode            Remarks
<ESC>     command mode               (get back to the command mode from any editing mode)
i         "insert" editing mode      (start inserting before the current position of the cursor)
DO NOT PRESS ANY OTHER KEYES IN THE COMMAND MODE. THERE ARE MORE COMMANDS AND MODES IN THE COMMAND MODE!

Copying, cutting and pasting (in the command mode):
v      start highlighting text. Then, move the cursor to highlight text
y      copy highlighted text
x      cut highlighted text
p      paste text that has been cut/copied

Saving and quitting (from the command mode):
:w               write (=save)
:w filename      write the contents to the file "filename"
:x               save and exit
:q               quit (it won't let you if changes not saved)
:q!              quit discarding changes (you will not be prompted if changes not saved)