You and your boyfriend will pay less taxes for 2009 if you wait to get married until next year.
If you do remain unmarried and file separate returns, your boyfriend cannot claim one of your children on his return. Your 3 children are your qualifying children, and cannot be claimed by your boyfriend. He will, however, claim his own son, assuming that son lives with him for most of the year.
More than likely, only one of you will qualify to file as head of household and the other will need to file as single. In order to file as head of household, you must pay more than half the cost of keeping up a home for a qualifying person. The costs that are included for purposes of keeping up a home are expenses such as rent, mortgage interest, real estate taxes, insurance on the home, repairs, utilities and food eaten in the home. When you total all those expenses together, whichevery one of you paid more than half of those expenses is the one who can file as head of household. Since you have the higher income, it would be more to your advantage to claim this status since it is a higher deduction.
Since your boyfriend's income is only $20,400 for the year and he has one qualifying son of his own, his income is low enough that he actually qualifies for the earned income credit. This is a credit which is above and beyond any taxes he owes. With his income of $20,400, just using the standard deduction and giving him credit for two personal allowances (himself and his son), his taxable income would be reduced to $7,400 for the year. He would owe tax on this amount of $740. However, at the same time he qualifies for an earned income credit of approximately $2,160, so he would actually end up not only not owing any tax, but he would get a refund for the difference of $1,420. If you got married this year and reported your combined income as a married couple, he would no longer qualify for this credit, and would end up paying higher taxes.
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Thank you johnsnurse