Should I file as a resident alien? I am single. 9/20/2013-1/12/2014 on J1 non-degree student. 7/31/2014-8/31/2020(expected) F1 student (12/18/2014-1/12/2015 outside US).

9/20/2013 was my first day in the U.S. Thank you so much!

Since you entered US  in 2013, and subsequently changed from J to F visa, your total exemption would still be 5 Calendar years. Therefore, your exemption status is over in 2017 . 2018 you start counting days present and thus you are a  resident for tax purposes for the year. This means you file a form 1040 return.

Thank you so much!