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Form 1042-S for foreign resident and owner of trade account
As you have indicated, the 1042-S reflects no income or withholding.  As such, you are not required to file a US tax return unless you have other income from the US.  Gains on stocks are not reported until realized upon sale. FYI, your activity does not fit the requirements of being engaged in a trade or business activity.  While the definition is documented in several places of the Code, I have attached Publication 519 which most accurately represents the application to your situation as you have described it. https://www.irs.gov/publications/p519/ch04.html#en_US_2016_publink1000222308