I am a avid user of Microsoft word, but today when I was typing I wanted to add a sentence to the middle of the paragraph with out erasing what I had written. When I started to type I looked up and saw what I was typing then was replacing what I had written. How can I stop this from happening? I really need to know because this is pissing me off. Please help me!

press your <Insert> key on your keyboard. It toggles the insert/overwrite mode on/off. You've apparently turned insert off and put Word into overwrite mode.