Command mode to Insert Mode

r to replace the character at the cursor

c change operator
c [count] motion
This command requires motion
ce to change all the way to line
Basically, deletes the word and places you into insert mode

Insert will put cursor BEFORE current position

I Insert at the beginning of the line

A Insert at END of the line

O New line ABOVE current cursor

Append will put cursor AFTER current position

A Append at the end of the line

o New line BELOW current cursor