As a J-1 visa holder, you are considered as 'exempt' by IRS for two years from the time you first entered the United States. It means they don't count your days during the time you live in the US. You are treated as a nonresident for the first two years.
If a person who is on J-visa, but has resided in US longer than the two years period of time, he or she may be considered as a resident if starting on the 3rd year, they reside in the US for at least 183 days by meeting the Substantial Presence Test.
In your situation, you are a nonresident for 2013 and 2014 and required to file a form 1040-NR for those two years. During the tax year of 2017, if you met the Substantial Presence Test and filing requirements, you are required file a 2017 US tax return.
If you provided dependent personal services performed as an employee (received a W-2) and obtained less than $10,000 in the United States, you are exempt from U.S. tax. You are advised to file a Form 8833 along with your tax return to claim the tax treaty benefit. Please see attached screenshot.
[Edited 1/13/2018 4:32 PM EST)
