Most newcomers keep a bank account at home. That is legal. But once you are a US person for these rules, you may have to report those accounts every year. Reporting is not a tax; the penalties come from not reporting.
US citizens, green card holders, and anyone who is a resident for tax (see the tax residency guide). People who are nonresidents for tax, such as most F-1 students in their first 5 years, usually do not file the FBAR.
You must file if:
The total of all your foreign financial accounts was more than $10,000 at any time in the year.
Count all accounts together: savings, checking, fixed deposits, some investment and pension accounts, and accounts where you can sign for someone else (for example, a parent's account). Use the highest balance of each account during the year.
Example: savings at home reached $6,000, and a fixed deposit was $5,000. Total $11,000: you must file, even though each one is under $10,000.
Collect for each account: bank name and address, account number, and the highest balance in the year.
Convert to dollars using the Treasury's year-end exchange rate (search "Treasury reporting rates of exchange").
Go to bsaefiling.fincen.treas.gov and choose the FBAR (Form 114).
File by April 15. If you miss it, there is an automatic extension to October 15.
The FBAR is filed with FinCEN, not with your tax return. It is free.
This form goes with your tax return. It covers foreign accounts and other foreign financial assets, such as foreign shares or certain foreign insurance and pension policies. For people living in the US, file it if your foreign financial assets were more than:
Single (or married filing separately): $50,000 on the last day of the year, or $75,000 at any time.
Married filing jointly: $100,000 on the last day, or $150,000 at any time.
FBAR and Form 8938 are separate. You may need one, both, or neither.
Other foreign items to know
Interest earned on accounts at home is taxable income for US tax residents, even if it stays abroad.
Foreign mutual funds can have very complex US tax rules. Ask a tax professional before you buy or keep them.
The IRS has procedures for people who missed forms without bad intent. Talk to a tax professional before filing late forms.
Thinking the limit is per account. It is the total.
Forgetting small or old accounts, or accounts you can sign for.
Thinking you need to close your accounts at home. You only need to report them.