I have a problem using the external screen on my acer 5315

I'm assuming you mean you get no picture on the external monitor.

Most laptops have a blue "function" key. some of the other keys have additional functions when this blue key is pressed as well; one of them is to change the display usage. Usually, this can be:
* Internal only
* external only
* internal & external showing same picture
* internal & external as extended desktop
Repeated pressing of the relevant key will cycle round the available display modes. Once you find the one you want, you may have to adjust Windows display options - right click on the desktop and click "properties" then go to the Settings tab.