Ch9. Filing Your Taxes — Form 1040, Deadlines, Extensions, and Amended Returns

The US Tax Filing System

The US operates on a self-assessment system: you calculate how much you owe, file the return, and pay (or receive a refund). Employers and payers withhold estimated taxes throughout the year, but the annual filing reconciles everything.

Key dates:
  January 31:     Employers send W-2s; 1099 payers send most 1099s
  April 15:       Federal return due date (most years)
                  Q1 estimated tax due (self-employed)
  June 15:        Q2 estimated tax due
  September 15:   Q3 estimated tax due
  October 15:     Extended return deadline (if extension filed by April 15)
  January 15:     Q4 estimated tax due (for prior year)

Form 1040 — The Individual Tax Return

The Form 1040 (with attached schedules) is the master document. Here is how it flows:

Part 1: Filing Information
  → Name, SSN, filing status, dependents

Part 2: Income
  → W-2 wages, 1099 interest, dividends, Schedule C, capital gains,
     IRA/pension distributions, Social Security benefits, other income

Part 3: Adjustments to Income (Above-the-Line Deductions)
  → Student loan interest, educator expenses, HSA contributions,
     SE tax deduction, alimony (pre-2019), IRA deduction
  → Result: Adjusted Gross Income (AGI)

Part 4: Standard or Itemized Deduction
  → Choose the larger:
     Standard deduction (2024: $14,600 single / $29,200 MFJ)
     Itemized (Schedule A): mortgage interest, state/local taxes, charitable gifts, medical

Part 5: Taxable Income and Tax
  → Apply tax brackets to compute tentative tax

Part 6: Credits
  → Child Tax Credit, Child and Dependent Care, Education Credits,
     Earned Income Credit (EITC), Foreign Tax Credit, etc.

Part 7: Other Taxes
  → AMT, SE tax, NIIT, recapture of credits

Part 8: Payments
  → Withholding from W-2/1099, estimated tax payments

Result: Refund or Balance Due

Key Schedules

Schedule	Purpose
Schedule A	Itemized deductions (mortgage interest, state taxes, charitable)
Schedule B	Interest and ordinary dividends > $1,500
Schedule C	Business income / loss (sole proprietor)
Schedule D	Capital gains and losses
Schedule E	Rental income, S-Corp / partnership K-1 income
Schedule SE	Self-employment tax computation
Form 8949	Individual capital asset transactions (feeds into Schedule D)
Form 2210	Underpayment penalty for estimated taxes

Filing Status — Choosing Correctly

Your filing status affects tax brackets, standard deduction, and credit eligibility.

Single:
  → Unmarried on December 31

Married Filing Jointly (MFJ):
  → Usually lowest combined tax
  → Both spouses jointly liable for any issues

Married Filing Separately (MFS):
  → May benefit in limited situations (student loan IBR, liability separation)
  → Loses many credits (EITC, education credits)

Head of Household (HOH):
  → Unmarried with a qualifying dependent
  → Better brackets and higher standard deduction than Single

Qualifying Surviving Spouse:
  → Used up to 2 years after spouse's death if you have a dependent child
  → MFJ rates and standard deduction

Estimated Taxes for Self-Employed and Investors

If you expect to owe $1,000 or more in federal tax not covered by withholding, you must pay quarterly estimated taxes.

Safe Harbor Rules (avoid underpayment penalty):
Option A: Pay 100% of last year's tax liability (110% if prior-year AGI > $150,000)
Option B: Pay 90% of this year's actual liability

Quarterly deadlines:
  Q1 (Jan–Mar income): April 15
  Q2 (Apr–May income): June 15
  Q3 (Jun–Aug income): September 15
  Q4 (Sep–Dec income): January 15 of following year

Payment methods:
  → IRS Direct Pay (free, from bank account)
  → EFTPS.gov (Electronic Federal Tax Payment System)
  → Credit card (processing fee applies)

Extensions

Form 4868 — Automatic 6-Month Extension

Deadline to file: moves from April 15 → October 15
Deadline to PAY: still April 15

Filing an extension does NOT extend the payment deadline.
Underpayment accrues interest and potential penalties from April 15.

To avoid penalties: pay your estimated balance due by April 15
even if you're not ready to file.

Amending a Return — Form 1040-X

If you made an error (missed a deduction, received a corrected 1099, etc.), file an amended return.

Form 1040-X:
→ Can be filed within 3 years of the original filing deadline
   or 2 years from when you paid the tax (whichever is later)

Common reasons to amend:
✓ Missed a significant deduction (HSA, student loan interest)
✓ Received a corrected 1099 or K-1
✓ Changed filing status (married / divorced)
✓ Claimed an incorrect credit
✓ Forgot to report income (proactive correction)

Do NOT amend for:
✗ Math errors (IRS corrects automatically)
✗ Missing W-2s (IRS will contact you)

What to Do If Audited

Audit types:
1. Correspondence audit (most common)
   → IRS sends a letter requesting documentation for a specific item
   → Respond with supporting documents within the deadline
   → Most resolved by mail

2. Office audit
   → Meet at an IRS office to discuss specific issues

3. Field audit
   → IRS agent visits your home or business
   → Rare; typically for complex or large returns

Red flags that increase audit risk:
✓ Unusually high charitable deductions relative to income
✓ Large home office deductions (especially on W-2 income)
✓ Schedule C with consistent large losses
✓ Income inconsistent with assets
✓ Round-number estimates (use exact figures)

Rights as a taxpayer:
→ Right to representation (CPA, EA, tax attorney)
→ Right to appeal IRS determinations
→ Right to privacy
→ Right to confidentiality

Record Keeping

Keep tax records for:
  General rule:   3 years from filing date (IRS audit window)
  Unreported income (> 25% of gross): 6 years
  Fraud / no return filed: indefinite
  Investment records: until 3 years after you sell the asset
  Home improvement records: until 3 years after you sell the home

Documents to keep:
  → W-2s, 1099s, K-1s
  → Receipts for deductions (charitable, business, medical)
  → Brokerage statements showing cost basis
  → Prior-year returns (at least 7 years)
  → Records of large asset purchases and improvements

Practice Quiz

Q1. You file for an extension on April 15 but do not pay the balance you owe. What happens?

The extension only extends the filing deadline to October 15 — not the payment deadline. Interest accrues from April 15 on any unpaid balance, plus a failure-to-pay penalty of 0.5% per month (up to 25% of unpaid tax). Always pay as much as possible by April 15 even if filing late.

Q2. You are self-employed and expect to owe $8,000 in tax this year. Your prior-year tax was$ 5,500. Under the safe harbor rule, what is the minimum you must pay in estimated taxes to avoid the underpayment penalty?

Since the prior-year AGI was presumably ≤ $150,000, the safe harbor is 100% of last year's tax = **$ 5,500**. Paying at least $5,500 in estimated taxes (across all four quarters) avoids the penalty regardless of the actual current-year liability.

Q3. You received a corrected 1099-B in March showing $3,000 more in capital gains than you originally reported on your April return. What should you do?

File Form 1040-X (Amended U.S. Individual Income Tax Return) to report the additional $3,000 of capital gains, pay the additional tax owed plus interest, and include a copy of the corrected 1099-B. You have up to 3 years from the original due date to file the amendment without penalty.