Key Action
i Insert at cursor (goes into insert mode)
a Write after cursor (goes into insert mode)
A Write at the end of line (goes into insert mode)
ESC Terminate insert mode
u Undo last change
U Undo all changes to the entire line
o Open a new line (goes into insert mode)
D Delete contents of line after the cursor
C Delete contents of a line after the cursor and insert new text.
4dw Delete word
cw Change word
x Delete character at the cursor
r Replace character
R Overwrite characters from cursor onward
s Substitute one character under cursor continue to insert
S Substitute entire line and begin to insert at the beginning of the line
~ Change case of individual character
These commands are entered from command mode
Using vi too much will make you feel like this