UPDATED December 2018
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A. You are unmarried or “considered unmarried” on the last day of the year.
Note: HOH has new due diligence requirements for the 2018 tax year, so a preparer will ask tougher questions, and the IRS is more likely to question this status - make sure you have your ducks in a row.
It is CRITICAL that you pay attention to questions asked by software about where the child lives, how long, divorce etc. Failing to do this will cause a lot of trouble for both of you.
Final Points: Make sure you have your children's and ex (or soon to be ex) spouses SSN and birthday - you will need them for taxes!