Although since October (so three months out of the year) I did not pay 50 percent of expenses.
[I]...paid 100% expenses until October
Where did you live until October?
I support my children. I lived in an apartment with just me and my children until October.
Based upon your fact pattern, you can file as HOH for 2017 and claim the children.
Your BF will have to file as Single (unless he is still legally married to someone else).
Thank you! So I can't claim HOH if he does even though I lived without him for most the year?
HOH means one provided more than 50% of the support for the entire household for the ENTIRE YEAR. You meet that qualification for your household.
I assume BF does not have any of his own children living with him. Why does he think he qualifies for HOH? Did he live elsewhere prior to October? You and your kids are not related to him, so cannot qualify him for HOH in any case.)
BF cannot claim Head of Household based on you or your children living with him, even if you lived there for the entire year. None of you are related to him, so you cannot be qualifying dependents for him to file as HOH.
And in any case, only one person in a given household can file as HOH, because only one person could have paid "more than half" the household expenses. Of course your BF is disqualified regardless, as explained above.
BF does have his own child that was just the two of them in their household until October when we my children and I moved in.
In that case, you both might qualify. @TomD8 Your thoughts?
@SweetieJean I agree. If the BF paid over half the cost of keeping up a home in which he lived for over half the year with his own child who he can claim as a dependent, then yes, he too would qualify to file as HOH.
But be prepared to provide documentation if the IRS questions it, especially since you will be filing from the same address.