You must file a Form 1042-S even if you did not withhold tax under chapter 3 because the income was exempt from tax under a U.S. tax treaty or the Code, including the exemption for income that is effectively connected with the conduct of a trade or business in the United States, or you released the tax withheld to the recipient. For exceptions, see Amounts That Are Not Subject to Reporting on Form 1042-S, later.
How to report income on your tax return. In most cases, you will not need to report the income on your Form 1040 or 1040-SR because the income will be exempt from U.S. tax under the treaty.
Enter the amount for which treaty benefits are claimed on Schedule 1 (1040), line 8z. Enter “Exempt income,” the name of the treaty country, and the treaty article that provides the exemption. Combine the amounts reported on lines 8a through 8z on Schedule 1 (Form 1040) and enter on line 9. Then, combine the totals from Schedule 1 (Form 1040), lines 1 through 7 and 9 and enter the total on line 10. Then, enter the total from Schedule 1 (Form 1040), line 10, on Form 1040 or 1040-SR, line 8.
If the income has been reported as taxable income on a Form-W-2, Form 1042, Form 1099 or other information return, The scholarship or fellowship recipient who are resident alien for the tax year should generally report the income on line 1 of Form 1040. Enter the amount as a negative number for which treaty benefits are claimed on Schedule 1 (1040), line 8z. Enter “Exempt income,” the name of the treaty country, and the treaty article that provides the exemption.
但是这里提到的 The scholarship or fellowship recipient 并不 apply，根据 1042-S box 1 的 income code (我是 20 Compensation during studying and training）来看，与 16 Scholarship or fellowship grants 并不一致，所以对这个不是很确定
If Form 1042-S lists federal tax withheld in box 7 or state tax withheld in box 21, you can enter this data on a Form 1099-MISC in the program. You must paper file your return in this instance, including Form 1042-S with your tax return.
我RA 5年没报过1042-S，没问题的。
（也许）