I'm a resident of Germany (Bavaria) and I’ve submitted my taxes via smartsteuer for three years. Smartsteuer warned me that for the last two I need to pay to Finanzamt a specific amount of money (Nachzahlung). I’ve checked why exactly and, after short yet intense investigation came up to conclusion that, hélas, it is what it is, and all is justified.
However I’ve received a letter from Finanzamt where they are requesting way bigger amount compared to that Smartsteuer warned me about. Like a big-big amount, like a noticeable part of my year income.
I've checked the "Zusammenfassung der wichtigsten Daten" which is basically a comparison table and it looks like the reason is that somehow they treated me as a single person, not a married one (thus paying taxes throughout whole year in a wrong category) - at least for 2017 I see in the report "Summe der Einkünfte" for both me and my wife but starting from 2018 I don't see any data on my spouse.
In 2018 and 2019 however, I've been married - and filed petition with my wife-at-the-time of course.
My question would be: What is exact set of steps I should take in order to make this claim reconsidered? Should I pay and the appeal, should I just write a letter or I should take some more active measures to secure my position?