The foreigner who has the U.S. source income must file the federal tax return with the IRS on time. Taxpayers may be subject to the penalty if they have not filed a timely return or did not pay taxes they owed. The general rule is that IRS requires taxpayers to file last six years of tax returns to be in good standing.

Why should you file back taxes?

It’s important to file accurate back returns for several reasons:

1. Stop late filing and payment penalties
   
   If you owe taxes and have not filed a timely return, you may be subject to the failure to file penalty (5% of the unpaid taxes for each month that a tax return is late, maximum of 25%), unless you can show reasonable cause for failing to file timely. You may also be subject to the failure to pay penalty (0.5% of the unpaid taxes for each month, maximum of 25%). If both a failure to pay and a failure to file penalty are applied in the same month, the failure to file penalty will be reduced by the amount of the Failure to Pay Penalty applied in that month.
   
   Interest is charged on taxes not paid by the original due date, even if you have an extension of time to file or pay. Interest is also charged on penalties.
   
   
2. Claim a refund
   
   There is usually no penalty for failure to file if you are due a refund. But you risk losing your refund if you don't file your return. In most cases, an original return claiming a refund must be filed within three years of its due date for the IRS to issue a refund. Generally, after the three-year window closes, the IRS can neither send a refund for the specific tax year. nor apply any credits, including overpayments of estimated or withholding taxes, to other tax years that are underpaid.
   
   
3. Avoid issues obtaining loans
   
   Loan approvals may be delayed if you haven’t filed your return. For example, financial institutions and mortgage lenders may require income verification that includes copies of filed tax returns submitted when you buy or refinance a home, get a loan for a business, or apply for federal aid for higher education.
   
   
4. Statutes of limitation
   
   As a general rule, tax on an income tax return must be assessed within three years of the filing of that tax return. When the statute of limitations expires, the IRS can no longer assess additional tax, or take collection action. The normal the statute of limitations for IRS to assess and collect any outstanding balances does not start until a return has been filed. In other words, there is no statute of limitations for assessing and collecting the tax if no return has been filed.
   
   
5. Protect social security benefits
   
   If you are self-employed and do not file your federal income tax return, any self-employment income you earned will not be reported to the Social Security Administration and you will not receive credits toward Social Security retirement or disability benefits.

How many years back should you file?

IRS Policy Statement 5-133 (P-5-133), Delinquent Returns—Enforcement of Filing Requirements, provides a general rule that the enforcement period is not to be more than six years. IRS management approval is necessary if the enforcement activity is less than or exceeds the six-year period. However, the extent to which delinquency procedures will be enforced will depend upon the facts and circumstances of each case, and by reference to factors ensuring evenhanded administration of staffing and other Service resources. Enforcement for longer or shorter periods may be used when consideration has been given to:

1. The taxpayer’s prior history of noncompliance.
2. The existence of income from illegal sources.
3. The effect upon voluntary compliance.
4. The anticipated revenue in relation to the time and effort, required to determine tax due.
5. Any special circumstances existing in the case of a particular taxpayer, class of taxpayer, or industry, or which may be peculiar to the class of tax involved.

 
How to file tax returns for previous years?

Regardless of your reason for not filing, file your tax return as soon as possible as the following steps.

1. Gather information
   
   The first step is to gather tax information from the year you need to file a return for. You can also require tax documents from the IRS. You can fill out the Form 4506-T, Request for Transcript of Tax Return, to request a transcript of your tax return information and income information on file, even if you haven’t filed a tax return. The IRS will send the information it has on record, including information found on forms such as W-2s, 1099s, and 1098s. It's essential to prepare an accurate return that matches IRS records. Without this match, the IRS can question the accuracy of your return.
   
   
2. Complete and file the tax returns
   
   Once you have all the documents you need, be sure to use the tax forms from the year you're filing. You can use the tax software or download the forms you need on the IRS website. Then you need to print and mail the returns to the IRS. The IRS may take longer to accept back tax returns.

Reference:
https://www.irs.gov/irm/part4/irm_04-012-001
https://www.irs.gov/pub/irs-utl/oc-havent-filed-your-tax-return.pdf
https://www.irs.gov/payments/failure-to-file-penalty
https://www.irs.gov/payments/failure-to-pay-penalty
https://www.irs.gov/individuals/dont-lose-your-refund-by-not-filing
https://turbotax.intuit.com/tax-tips/irs-tax-return/how-do-i-file-back-tax-returns/L535BxMms
https://www.irs.gov/pub/irs-utl/oc-havent-filed-your-tax-return.pdf
https://www.irs.gov/newsroom/help-yourself-by-filing-past-due-tax-returns
https://www.irs.gov/businesses/small-businesses-self-employed/filing-past-due-tax-returns