I AM UNABLE TO GET THE FUEL CAP TO SCREW DOWN

Sounds like the cap has become cross threaded or has some sort of build up in the threads on the saw or on the cap. Try cleaning off all threads on saw and cap with a little two cycle oil on a rag, leaving a little oil film on the threads when you are done wiping them down. If that is not the case and does not fix the problem I recommend you take it to a repair shop and they can sell you a new cap for a couple of bucks then you can verify if the new cap is the solution. Good luck.