How the IRS, the Tax Year, and the Progressive System Fit Together
By the end of this lesson, you'll understand:
A lot of tax confusion comes from not knowing the basic shape of the system, what the IRS is actually responsible for, how a "tax year" becomes a return you file months later, and why the rules seem to shift slightly every year. Understanding the shape of the system makes every individual rule easier to place.
The Internal Revenue Service (IRS) is the federal agency responsible for administering and enforcing the federal tax code, collecting tax revenue, processing returns, issuing refunds, and enforcing compliance. It does not write tax law; Congress does, through legislation the IRS then implements via regulations and guidance.
What to check: When a tax rule changes, it's usually the result of new legislation, not the IRS acting on its own, worth remembering when a rule feels arbitrary or confusing.
The U.S. uses a progressive income tax system, meaning different portions of your income are taxed at different rates as your income rises, rather than one flat rate applying to your entire income. This concept, and the myth it creates, gets a full lesson next, for now, the key idea is that the system is designed to tax higher portions of income at higher rates, not your entire income at one rate.
What to check: Note that this progressive structure applies to federal income tax specifically, payroll taxes like Social Security and Medicare work differently, as covered in the previous lesson.
The tax year is the calendar year your income and taxes are based on (for example, 2025). The filing season is the following spring, generally opening in late January and running through the mid-April deadline, when you file the return covering that prior tax year. This gap is why people sometimes confuse "this year's taxes" with the return they're actually filing.
What to check: When you hear a tax figure, a bracket, a deduction amount, confirm which tax year it applies to, since these figures are adjusted annually for inflation and legislation.
Adjusted Gross Income (AGI) is your total income from all sources minus specific adjustments (like certain retirement contributions or student loan interest). AGI is a foundational number used to determine eligibility for many deductions and credits, and it appears on nearly every subsequent form and calculation in this course.
What to check: Once you file a return, locate your AGI on the form, it's a number worth knowing, since many credit and deduction eligibility rules are based on it, not your gross income.
Lena earned $58,000 in wages in 2025 and contributed $2,000 to a traditional IRA, which is one of the specific adjustments allowed. Her Adjusted Gross Income for the year is $56,000, not her full $58,000 in wages.
When she later checks her eligibility for a tax credit that has an AGI limit, she uses the $56,000 figure, understanding this distinction prevents her from incorrectly assuming she doesn't qualify based on her gross wage figure alone.
The IRS decides tax rates and writes the tax rules.
Congress writes tax law through legislation; the IRS administers and enforces it. Annual adjustments to brackets and deduction amounts are often inflation adjustments built into existing law, not new IRS decisions.
Most are adjusted annually for inflation under existing law, though Congress can also pass new legislation that changes rates, brackets, or deduction amounts more substantially, as happened with recent tax legislation.
No. AGI is calculated first, and then further deductions (standard or itemized) are subtracted from AGI to arrive at your taxable income, which is covered in an upcoming lesson.
Filing season typically opens in late January for the prior tax year, though the exact date is announced by the IRS each year, worth confirming rather than assuming a fixed date.
Look up the current filing season's opening date and deadline for the tax year you'll be filing next, and mark them on your calendar.
With the system's basic shape in place, the next lesson, TXS103: Understanding Tax Brackets and Marginal vs. Effective Tax Rate, tackles one of the most commonly misunderstood parts of the whole system.
That's where Financial Confidence becomes your personal tax system explainer.
Financial Confidence can help you track current tax year figures and deadlines, calculate your AGI, organize the adjustments that reduce it, and keep your filing timeline on schedule.
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