According to the Massachusetts Form 1 instructions, you are considered single if, as of December 31, 2016, "you were legally separated under a final judgement of the probate court". Please refer to page 20 of the instructions under the heading, "single" for additional clarification.
http://www.mass.gov/dor/docs/dor/forms/inctax16/f1-nrpy/form-1-instructions.pdf
You can file as head of household for Massachusetts if you meet the requirements to file as head of household on your federal tax return. The federal requirements allow you to qualify for this status even if you were still legally married as of the end of 2016. Please refer to the following FAQs to determine if you meet these requirements.
https://ttlc.intuit.com/replies/4206137
If you were legally separated as of the end of 2016, then you would need to meet the requirements in the following FAQs in order to file as head of household.
https://ttlc.intuit.com/replies/3288637 https://ttlc.intuit.com/replies/4791157
The Massachusetts instructions do NOT state that you actually have to have filed as head of household on your federal return in order to file as head of household on your Massachusetts return.
Your spouse, if still legally married to you as of the end of 2016, can file as married filing jointly, married filing separately, or head of household (rules that apply to individuals who are married). If you are legally separated as of the end of 2016, your spouse can file as single or head of household (rules that apply to single taxpayers). Keep in mind that if both of you file as head of household, you cant have the same qualifying person.
