vi Reference    from     http://acs.ucsd.edu/info/vi_tutorial.php

Undo Commands
u              Undoes the last command ONLY
U              Undoes all changes to the current line you are working on

Insert Commands
i              Inserts text to the left of the cursor
I              Inserts text the beginning of the line

Append Commands
a              Begins inserting after the character (append) on which the 
               cursor is positioned.
A              Begins inserting at the end of the current line, no matter where
               the cursor is positioned on that line.
               
Open Commands
o              Begins inserting after the character (append) on which the 
               cursor is positioned.
O              Begins inserting text on a new, empty line

Spell Check
:w !spell -b  Spell Check

Quitting
:wq           Write Quit
:q!           Quit