Did she get any 1099R? On 2019 1040 line 4b is the taxable amount of IRA withdrawals. Did she take any IRA withdrawal or conversions or rollover? It's possible to have zero taxable amount. It might also say ROLLOVER by line line 4b.
Line 5a and 5b are for Social Security. Did she get a SSA-1099?
Up to 85% of Social Security becomes taxable when all your other income plus 1/2 your social security, reaches:
Married Filing Jointly: $32,000
Single or head of household: $25,000
Married Filing Separately: 0
However you each filed in 2019 should not affect filing 2020. Don't know about the bills from the IRS. You have to ask them. Could it be interest for paying the tax due late? If the IRS changed your return the letter should say what the changed. There should be a LTR or CP # up at the top. What is it? Or is it from your state? Maybe a state tax due didn't get paid.
Here's some general info on getting married. Did you both use Turbo Tax last year?
You can not transfer from or combine 2 returns or accounts. Just pick the return for the spouse that has the most complicated return to enter or has any depreciation or business or more investments. Or use the account for whoever you want to be listed first going forward (by the way you can not change the order of the names). Then add the other spouse's name, ssn and info to it.
You should usually file a Joint return which is only 1 return combined for the both of you. Make sure to stick with this one account for the future. Just ignore the other spouse's account.
Getting Married FAQ
https://ttlc.intuit.com/community/tax-filing-status/help/what-does-getting-married-mean-for-my-taxes...
And info on Getting Married…..
http://turbotax.intuit.com/tax-tools/tax-tips/Family/Getting-Married/INF12006.html
Is it better to file Joint or Separately?
https://ttlc.intuit.com/questions/1894449-is-it-better-for-a-married-couple-to-file-jointly-or-separ...
Compare Joint to Married Filing Separately
https://ttlc.intuit.com/questions/2895312-how-can-we-compare-married-filing-jointly-with-married-fil...
