Partner and I no longer live together. He had us on his health insurance. We had to sign a paper saying we were common law. Do we have to file a joint return?

[DanielV01]

No, you don't have to file a joint return, but you might be married for tax purposes.  It depends on your state.  If you were together in Colorado, Iowa, Montana, Kansas, New Hampshire, Utah, South Carolina, Texas or Washington DC, your union may well be recognized as a marriage because those states do recognize common-law marriages, and if your union is recognized as a marriage in the state you were living in, you are married for tax purposes as well.  This would mean that your options to file would be Married Filing Joint, Married Filing Separately, and if you were apart for more than the last 6 months of the year, and you have a child you can claim, you may also use the status of Head of Household.