The tax filing status as married or single is determined as of the last
day of the year. So, if the divorce remains pending as of December 31st,
the parties are still married for tax purposes. If the divorce is granted
during the tax year, then the parties’ marital status is probably
single as of December 31st.
Typically, parties cannot be forced to sign a joint tax return. In most
circumstances, it is usually more advantageous for parties to file a joint
return than to file separately. The advice of a good CPA to give each
spouse independent advice makes sense.