My tax status is Resident Alien. Last year, I forgot to claim the tax treaty. I wonder if I could claim the previous year's tax treaty this year using TurboTax?

[ErnieS0]

It depends. You can claim a tax treaty exemption if you qualify for a treaty exemption in 2022. You can’t claim last year’s exemption on this year’s return.

You can amend your 2021 return to claim the exemption.

See How to Correct Federal Tax Returns

[pk]

@guoyungui , agreeing with  @ErnieS0 ,  you need to tell us  1. your visa status  ( which visa F-1/ H1/J1 or ??); 2. when did you enter the country ( and obviously why you think you are a tax resident) ; 3. which country are you from  and 4. exactly which  treaty condition are you forgot to assert for 2021 tax year.  As you are undoubtedly some countries have time limitations , some have status / intent  limitations for assertion of treaty benefits.

I will circle back once I hear from you

[guoyungui]

Thanks @pk @ErnieS0 
My visa is F-1. I have been in the States since 2015 to pursue my degree. According to the IRS, I'm RA for tax purposes since 2020.
There is a 5000$ tax-free income treaty between the two countries. Last year, I forgot to claim it when I did the tax return. Based on the information I found on the IRS website, I could use this tax treaty as long as I am a student.

So, to amend last year's tax return, I know I need to file the 1040-X form, what other forms should I file? And do I need to send my original tax return form along with the 1040-x to the IRS?
Appreciate your help.

[DaveF1006]

To clarify, what country are you from? Also in 2021, did you file a 1040 or a 1040 NR?

[pk]

@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 ?