Final answer:
True, married taxpayers who file a joint return cannot file separate returns for that year after the filing due date.
Step-by-step explanation:
The statement is true: once married taxpayers file a joint return, they cannot switch to filing separate returns for that tax year after the filing due date has passed. This rule is a standard guideline set by the Internal Revenue Service (IRS). The choice to file jointly is a binding election that the couple makes when they submit their tax return. It is significant because it can affect the tax rates and deductions applied to the combined income, as well as potential responsibility for the accuracy of the information reported and the liabilities for any taxes owed.