Vim 101: Entering Insert Mode
Top ten coolest ways to enter into insert mode
a - append after cursor
A - append at end of line
i - insert before cursor
I - insert before beginning of the line
o - open a new line below current line
O - open a new line above the current line
s - substitute a character
r - replace a character
cc - delete the whole line and enter into insert mode
C - delete forward of cursor and enter into insert mode
If you want to learn more about inserting text checkout
:help inserting or
:help A
:help cc