Can a married taxpayer file as single?

Usually no. If the taxpayer is considered married for federal filing-status purposes, single status is not available just because the taxpayer checked a box or lives separately from a spouse.

The preparer should first determine whether the taxpayer was married on the last day of the tax year, whether there was a final divorce or legal separation decree, and whether any special head of household rule might apply. If the taxpayer is still considered married and no head of household exception applies, the usual choices are married filing jointly if both spouses agree or married filing separately.