Hello, I am very low tech compared to most users here. I know there are threads answering this very question, but I cannot comprehend the jargon being used. Some background: I know how to connect a computer with Windows just fine. I know the little Fn then hit the mini screen thing(you can sense my lack of knowledge by the word choice) which WAS f4 on my computer before my hard drive died suddenly. My brother in law replaced the hard drive with a spare he had. It is Linux, and the f4 button no longer does anything. I tried all the other f buttons and nothing comes up giving me the option to choose screen only/duplicate/etc. I used my brother's computer the other day on the same setup and it worked fine(Windows) so I know the system in place at the location is not faulty. Please help in the simplest terms you can find in your hearts. Thank you!

Plug your projector in and go to display in the control panel and select either mirror or extend.

which linux version are you using

The specific method to do this will vary depending upon which Linux you are using. Most times, the name is on the screen when you boot the machine. There are hundreds of different Linux distributions so that would be the first step to get help.