Many of the games on this site are in a zip file when I download them. While in the zip, I double-click the jar and it either runs for a second, or it says there was an error running java. Some rarely work, but most don’t. Am I not supposed to start the jar?
You’ll need to run the jar from your OS’s equivalent of the command prompt (if not on Windows), and ask us specific questions using the stack trace that the prompt will give you. There generally isn’t a one size fits all answer for these kinds of issues.
You could also extract the file and see if that helps
Most executable files won’t run unless extracted.
probably because there are other files that the game needs in the archive