VIM useful commands must know them.

yy              : cory the current line.
p               : paste.
0(number )      : move cursor to the front of the current line.
$               : move cursor to the end of the current line.
u               : undo
ctrl + r        : redo
N<command>[ESC] : repeat.
                   eg:
                   10p : past 10 times
                   10ohello<ESC>  : write 10 line hello
set number      : display the line number.
gg              : move cursor to the first line
G               : move cursor to the last line
15G             : move cursor to the 15th line
*               : move cursor to the next same word
#               : move cursor to the last same word
%               : move cursor to the peer [{(
w               : move cursor to the front of the right word.
b               : move cursor to the front of the left word.
e               : move cursor to the last letter of the current word.
y$              : cory the current line from the current cursor position.
ye              : cory the current word from the current cursor position.
gU$             : change the current line to upper case
gUe             : change the current word to upper case
gu$             : change the current line to lower case
gue             : change the current word to lower case

Vim Usage:
-- cursor mover
n+       // move the cursor to the next n lines.
n-       // move the cursor to the pervious n lines.
nG       // move the cursor to line n.
G        // move the cursor to the last line.
--delete and undo
x        // delete the current char
nx       // delete n chars from the current char
dd       // delete the current line.
ndd      // delete n lines from the current line.
u        // undo the previous command.
U        // undo all the pervious commands.
--set the line number
:set nu     // set the line number display.
:set nonu    //cancel the line number display.
--copy and paste
yy      // copy the current line.
nyy     // copy from the current n line.
yw      // copy from the current cursor to the end of the word.
nyw     // copy n words from the current cursor.
y^      // copy from current cursor to first letter of the current line.
y$      // copy from current cursor to the end letter of the current line.
p
--replace
:s/old/new         //use new replace the first old in current line.
:s/old/new/g       //use new replace all the old in current line.
:n,m s/old/new     //use new replace the first old from line n to line m.
:n,m s/old/new/g   //use new replace all the old from line n to line m.
:%s/old/new/g      //use new replace all the old in the document.