The backlight on my laptop died so I got another monitor and hooked it up. I extended my windows desktop to the external monitor and it worked fine, except that there's nothing on the external monitor's screen except my wallpaper. Turns out it's working like two screens set up side by side...so if I scroll my mouse over to the left of my laptop screen, it disappears off the edge and appears on the screen of the external monitor. How can I make the external monitor function like a copy of the laptop monitor?

It sounds like you have 2 video controllers going at the same time. You might have to disable one. Look in the device manager or the BIOS.

First off most laptops have a hotkey combinations to toggle through the various monitor modes (such as fn F4.) Look on your function keys for an icon of a monitor. That is usually the one. This is a toggle function...hold down the fn key and then press the key indicated and observe the results. If it is not the result you want press the key indicated again. There may be as many as five or six modes to toggle through. If that does not work then right click on the desktop, select properties, settings, then it is usually under the advanced button to change these types (spanning/cloning of monitors) of settings.