The cursor is a block.
Sometimes one wants to insert text at the start,
one can do this with 'i'.
Other times at the end, this is done with 'a'.

This is extended to lines, by using the capital
letters 'I' and 'A'.

Sometimes you also want to add text on the line above/below.
There are likewise commands for this, 'O' and 'o'.

While typing, typos are common. For short words I know I just
re-type them. This is where 'C-w' comes in handy.
'C-u' does the same thing for lines.