18,000 and 26,000 for what? For the Standard Deduction?
For 2021 the standard deduction amounts are:
Single 12,550 + 1,700 for 65 and over or blind (14,250)
HOH 18,800 + 1,700 for 65 and over or blind
Joint 25,100 + 1,350 for each 65 and over or blind
Married filing Separate 12,550 + 1,350 for 65 and over or blind
Qualifying Widow gets the same as Joint.
You can file a Joint return the year your spouse died. For the next two years following a husband's or wife's death, the surviving spouse can file as a qualifying widow or widower if they have a qualifying child. That basically lets you continue to use the same tax brackets that apply to married-filing-jointly returns. After the year of death if you don't have a child you file as Single.
See the eligibility rules at the bottom of page 10 of IRS publication 501
https://www.irs.gov/pub/irs-pdf/p501.pdf
