a : append the text to the right of Cursor.
I : Insert the text to the left of the line.
A : Insert the text to the right of the line.
o : Open the line below.
O : Open line above.
r : Replaces the character under.
R : Replaces the text to the right.
: Replaces the character under the cursor with any no of characters.
S : Replaces entire line.