In your writing experience with Word there must be once or twice you modify something and the words followed by it just disappear. How does it happen? Well, you may toggle on themode unconsciously and don't know how to get it back.
Overtype mode is actually useful in some situations. But for us, it may be only annoying when you just want to modify one or two words. Therefore, it's better to know how to switch the mode.
1. Right click the book icon in the bottom, find the selection of Overtype. Here you can toggle on/off the mode as you like
2. You can also try an easier way. Press Insert key in your keyboard, once for enabling and twice for disabling. That's it, no more being disturbed by.