What is the rule for married people who do not live together?

Married Persons Who Live Apart
Even if you were not divorced or legally separated at the end of the year, you are considered unmarried if all of the following apply:

• You lived apart from your spouse for the last 6 months of the year. Temporary absences for special circumstances, such as for business, medical care, school, or military service count as time lived in the home.
• You file a separate return from your spouse.
• You paid over half the cost of keeping up your home
• Your home was the main home of your child, stepchild, or foster child for more than half the year.
• You can claim this child as your dependent or could claim the child except that the child's other parent can claim him or her under the rule for Children of Divorced or Separated Parents that begins on page 18 of Form 1040 Instructions.

You are considered unmarried for head of household filing status if your spouse was a nonresident alien at any time during the year and you do not choose to treat him or her as a resident alien. To claim head of household status, you must also meet Test 1 or Test 2 on page 13 of Form 1040 instructions.