Word 2003 , lose text when amending

[mark with a problem]

When I have typed a sentence and then try to amend it I lose the letters/text
in front of it.
If I was amending the following sentence I would lose the text in front of
the/ each time I tyed in a new letter or piece of text

eg.
I was walk/ng the dog

 

[garfield-n-odie [MVP]]

Press the Insert key on your keyboard once.