@guoyungui , given that you arrived in 2015 on a F-1, there were no prior entries with F-1 or J-1, generally your exempt ( from counting days present ) would have been for five Calendar years from entry date and thus 2019 would have been the end of your "Exempt" status. Therefore you are correct in that you are "Resident for Tax Purposes " for 2021 and should have filed your return on form 1040.
Since you did not mention the country you are from, but have stated $5000 tax free earning, I would assume that you are perhaps from China ( US-China Tax Treaty article 20 ). Since you forgot to assert this benefit for tax year 2021, suggest you file a 1040-X, recognizing this ( exclusion of $5000 from US taxes ) and attach a form 8833 naming the Tax-Treaty country and the article there-in. This is better/ simpler done in 1040-NR ( which cannot use anymore).
A simpler ( and correct ) way is to use Schedule-! --"Other income line z where you can list the exclusion and as a negative number ( i.e. - 5000) and under explanation write in US-China Tax Treaty article 20 .
Note that ( if we are indeed talking about China as your home country ) while China is most liberal in this benefit ( by not putting time limits ), any change of status ( say to H-1 or similar work visa ) or intent ( to be temporarily present ONLY for education / training ) would put this benefit at risk.
Does this help ? Is there more I can do for you ?
