2008 iMac not responding to any startup commands

Question

So a while ago my 2008 iMac had the flashing question mark folder thing happen to it (probably a bad factory reset). I suspect due to its age, the only startup command that worked was (from memory) option+R.

I then tried to install Mojave using a downloaded version I put onto my external hard drive, but it wasn't working because it wasn't a compatible OS for the iMac. Most recently I have put Mavericks (as I remember having that OS on it when it was working) on the hard drive, but now nothing is even showing up; not Utilities (which is what I was getting before), and none of the startup commands I've found have worked.

I would love to get it to work, and I don't think it'd be a hardware issue as it was still showing OS X Utilities on startup just the other day. What would be my next steps to getting this beast up and running again?

EDIT: I am only trying to get it to work because I'd love to install Linux on it. Would there be a way to do this without reinstalling OS X?

EDIT 2: I solved this. It was working when I was first installing OS X, but then it stopped, so at some point in the process of trying to install the OS my keyboard stopped working. Using another keyboard worked...