A spouse passing in 2018, when you file your 2018 tax return you can file as Married Filing Jointly.
For tax year 2019 if you have not remarried or have any dependent children you can only file as Single.
If you do have dependent children then you can file as a Qualifying Widower for two years after your spouse's passing. The Qualifying Widower entitles you to use joint return
tax rates and the highest standard deduction
amount (if you don't itemize deductions). It
doesn't entitle you to file a joint return.
Neither TurboTax or the IRS have a 2019 withholding calculator available at this time. The 2019 calculators should be available sometime in early 2019.
The 2018 TurboTax withholding calculator is here - https://turbotax.intuit.com/tax-tools/calculators/w4/
The 2018 IRS withholding calculator is here - https://apps.irs.gov/app/withholdingcalculator/
