IANAA, but if you’re a W-2 income earner your income tax is accounted for and paid each pay period, so the IRS already knows how much you’ve paid and they quickly know by the end of the year how much you ‘should’ have paid based on your tax bracket you landed in. Combining incomes for two spouses is just as easy, again if both are W-2 earners. From there, it’s just a matter of claiming deductions.
ETA: forgot to mention AGI, which is calculated based on last year’s income as well.
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