Final answer:
The form completed to show legal ability to work in the U.S. is the I-9 form. The W-4 form is for tax withholding, and the 1040 form is for filing your tax return.
Step-by-step explanation:
The form a person completes to demonstrate his or her legal ability to work and pay taxes on income is the I-9 form. This form is used by employers to verify an employee's identity and legal authorization to work in the United States. When you start a new job, you will likely also fill out a W-4 form, which determines the amount of federal income tax withholding from your paycheck. At the end of the year, you're responsible for filing a tax return, for which you might use a 1040 form, especially if your financial situation is complex. The W-2 form is given to you by your employer and shows the amount of taxes withheld from your paycheck throughout the year.