Dave was married in year 2008 so was filing jointly in 2008 and after his wife death he was filing single in 2009.
Joint tax return is a single tax return which is filed by a married couple that had married before the end of the tax year, on which their combined income, exemptions, credits and deductions are filed.
A joint return is filed with the Internal Revenue Service (IRS) on the Form 1040 (as of 2018) by two married taxpayers whose filing status is shown at that time of being married filing jointly (MFJ) or it is filed by a widowed taxpayer whose filing status is Qualifying Widow or Widower (QW).
A joint return allows the above given taxpayers to combine their tax liability as well.
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