When attempting to make corrections, the next letter is erased, what is wrong?

I get a dark black cursor when attempting to make corrections. As I make the corrections, the next successive existing character is then erased as I make corrections. Thus requiring me to rewrite entire paragraphs, what is wrong here?
Thanks…

Press Ins key.

Probably you are using the overwrite mode. As Mike suggested you only need to press the Insert Key to return to the insert mode and get the small cursor between two characters again.