i     - (insert)  right before (to the left of) the current character.
I     - (Insert)  right before first non-blank in the current line.
a     - (appaned) right after the current character.
A     - (Apppend) right after end of line.
o     - add new empty row below.
O     - add new empty row above.

Remember, ESC will take you from insert mode back to normal mode.