The filing status selected on a US return governs more of the calculation than most filers realize. It sets the thresholds that everything else is measured against.
Status sets the bracket thresholds
Each status has its own schedule of bracket thresholds, so identical income taxed under two statuses crosses into higher rates at different points.
The same income can therefore produce different liability with no change in earnings, deductions or credits, purely from where the bands begin.
This is why the status question is answered first. Nearly every subsequent line depends on the schedule it selects.
The standard deduction moves with it
Each status carries its own standard deduction amount, which is the figure subtracted from income before the brackets are applied.
A larger standard deduction shields more income entirely, which matters most for filers whose itemizable expenses fall below the threshold.
Because the deduction amounts are adjusted over time, the comparison between itemizing and taking the standard deduction changes from year to year.
Marriage can move liability either direction
Two earners filing jointly may find their combined income taxed more favorably than it would have been separately, or less, depending on how the incomes compare.
Similar incomes and very different incomes produce different results, because joint thresholds are not always double the single thresholds at every band.
Filing separately is available to married couples but restricts or eliminates several credits, so it is chosen for specific reasons rather than as a default.
Household status has strict tests
Head of household status offers more favorable thresholds than single status, but it requires meeting tests regarding dependents, residence and paying a share of household costs.
The tests are specific and are a common source of filing errors, since the everyday meaning of supporting a household is broader than the tax definition.
Qualifying and non-qualifying situations can look similar from the outside, which is exactly why the statutory tests exist in the detail they do.
Credits are gated by status and income together
Many credits phase out over an income range, and the range where the phase-out begins and ends depends on the filing status.
A household can therefore qualify under one status and not another at the same income, which is part of why the status choice is consequential.
These thresholds and tests are set in law and revised periodically, and individual circumstances determine which apply, so the actual determination belongs with a tax professional.