Couple married outside US and working on H1B. Any problem with filing taxes as single, if we (or one of us) would like to get green card in future?

As you do not present a marriage based case in which one party has status and is petitioning for the other party, the rules on proving a bona fide marriage including joint tax filings do not necessarily apply in your case. There would not likely be any immigration problem concerning the bona fides of your marriage as long as there is any other proof that you have a bona fide marriage in which you have lived together as man and wife. We do not deal in tax issues, however, and you may wish to look into whether filing single when married would be a violation of tax laws. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.