You owe money because the amount that is being withheld from your pay is not enough to pay the tax owed. It is as simple as that. To correct that, you can file a new W-4 withholding form with your employer, making sure that you specify to withhold as Single, and then claim 0 exemptions. That will withhold more. If that still does not withhold enough, you can always as to have an additional cash amount withheld each pay period.