we made a mistake and just realize that we filed as singles, what shall i do?
No need to panic.. It's an innocent mistake. These things happen. Just go to your Accountant and ask him/her to amend your taxes. No big deal. Taxes can be amended for up to 4 years. People do it all the time. You won't have to show any taxes to USCIS, but will be able to honestly answer all the questions on your N-400 and show good moral character.
I hope my answers were helpful.
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