If you were still legally married as of December 31, you are considered unmarried (and therefore eligible for Head of Household) if all 5 of these conditions apply:

    You won't be filing jointly with your spouse; and
    Your spouse didn't live in your home after June (temporary absences due to illness, school, vacation, business, or military service don't count); and
    Your home was your child's, stepchild's, or foster child's main home for more than half the year; and
    You paid more than half the costs of keeping up your home during the tax year; and
    You meet the qualifications to claim the child as your dependent, even if the other (noncustodial) parent is actually claiming the child as a dependent on their return. 

You can also be "considered unmarried" for Head of Household if your spouse was a nonresident alien at any time during the tax year and you're not treating them as a resident alien.

Married filers who are "considered unmarried" cannot file with Head of Household status if their dependent is somebody other than a child, for example a parent.