> I don't know what Sweden's tax code looks like, but at least here in the U.S., the IRS simply can't know a lot of the information necessary to calculate your taxes. How would it know how much you paid in daycare expenses, tuition and tuition interest, home mortgage interest, etc?
The same way that it already does know many of those things -- you know those student loan interest, mortgage interest, etc. statements you get at the end of the year that you rely on when filling out your tax forms, and that the tax forms refer to by form number? Yeah, not only are the people you paid required to send those to you, they are already required to file them with the IRS, as well.
There's some things that may not be amenable to this solution, but many taxpayers aren't effected by them and the workload if you reduced filing to addressing those would be minimal -- the IRS just sends you your precomputed "basic" return, with instructions on the handful of potential adjustments you might need to report and document and send back, and a deadline to do so if the basic return isn't all you need (along with the same deadline to pay any balance if the basic return is correct, or any remaining balance on the corrected return.)
Then, if nothing is filed before the deadline, the IRS sends out refunds (if applicable) or starts collection on any amounts due and not paid.