Yes, my understanding is that you are not permitted to file single when you are not single.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.
Tax issues should be addressed by complement tax counsel or tax accountant. The immigration aspect of your question is that separation and/or divorce should not affect the N-400 based on 5 years residency; provided the marriage through which permanent residence was granted was bona fide.
The person who posted this question, as well as anyone else who reads this response, should understand that the response is not, and should not be understood as, legal advice. Rather, it is legal information, based on the abbreviated facts presented. Immigration law is very complex, unfortunately, and immigration status adjudications are highly fact-dependent. The reader should retain an experienced immigration lawyer to analyze the facts specific to his/her particular situation to obtain “legal advice”; which this is not. Any answers offered on Avvo are of a general nature and informational only, and are not meant to create an attorney-client relationship.
Single filing status generally applies to anyone who is unmarried, divorced or legally separated according to state law. Unless you are legally separated, then you may not file as single.
If you are not legally separated, then you should speak to an accountant about which filing status will be most beneficial to you.
As to your N400, if you have had continuous presence for the five years, this should not be a problem, but the answer will depend upon the specifics of your application.
This does not constitute legal advice or the engagement of my services as an attorney.