First and foremost… please forgive my somewhat approximate English. (I'm French.) Oh and my loooooong sentences too (I need lots of words for little meaning when speaking foreign languages… plus I'm a writer.)
So this is an issue I've had to deal with some years ago, but unfortunately I forgot how to fix it. I'm currently working on some writings, using Microsoft Word (this is the family computer) and I'm having trouble which I think isn't related to software but to my keyboard settings. So, every time I try to INSERT text, I actually forward-delete and replace instead. I'm not sure these are the right words so let me try an explanation. If I write a sentence then want to edit it, by inputting new stuff in between words, well instead of "pushing" these words forward, I end up deleting them and replacing them with the new stuff.
I can't even start explaining how this is a pain for a writer, as I keep going back on sentences and editing them, sometimes editing whole pages of stuff. Now if I can't do it without deleting more stuff… I could end up not being able to input the stuff I want without losing the same amount of text. And said amount can reach huge proportions.
This wasn't happening five minutes ago so I'm guessing I accidentally input a key combination that triggered it. Now if someone could tell me what it is that I did (silly, heh ?) and how to undo it (Same key combination I guess ?), they'd have my eternal gratitude.
Just don't ask for a signed copy, I still need to put a good five more years of work on that stuff.
Pressing the Insert key (I believe it's Inser on French keyboards) is usually enough.
If that doesn't work, open Word and take a look at your status bar (bottom of the screen). In the English versions, one of them says OVR (Word 2003 and older) or Overtype (Word 2007 and newer). Cliking it it changes to Insert mode.