There are several different situations that may prevent you from saving a file:

  1. Sample files located on your computer are write protected. 
    This means that you can read (open) the file, but are prohibited from saving to that location. 
    When you first open a sample file, regardless of the program being used, go to File > Save As and save it to your own folder. This way, you have your own working copy. 
    You may make additional folders as needed. A key point here is to keep organized: Make folders for logical purposes. Name the files logically so you know what the assignment is by looking at the file name. If this is a file that you created from scratch, make sure you're saving to your own folder.
  2. If you downloaded the file, it might be in the temp or cache folder of the computer. Generally, files is this section cannot be modified, and hence an attempted save will fail. Do a Save As to a known location such as your document folder. In the event that you are able to save in these locations, the file will be deleted when you either close the application, log out, or shut down the computer. 
  3. If you opened the file directly from a compressed folder without unzipping the folder, then a save will probably also fail. Like the situations above, try doing a Save As to a different, known location. In practice, you should decompress the folder before opening the files for actual usage. The ability to open a compressed file is meant for examination of the file, not to work with the contents. 

Make sure you know the location where you're attempting to save the file. I have seen students save the file to a temporary folder: these folders are wiped clean by the system when you shut down, log out, or perhaps even just closing a particular program.

SAVE EARLY - SAVE OFTEN.

If you continue to have a problem, ask me.