http://www.uscis.gov/green-card/after-green-card-granted/maintaining-permanent-residence
Maintaining Permanent Residence
You may lose your permanent resident status (green card) if you commit an act that makes you removable from the United States under the law, as described in Section 237 or 212 of the Immigration and Nationality Act (INA). If you commit such an act, you may be brought before an immigration court to determine your right to remain a permanent resident.
Abandoning Permanent Resident Status
You may be found to have abandoned your permanent resident status if you:
Move to another country intending to live there permanently
Remain outside of the United States for more than 1 year without obtaining a reentry permit or returning resident visa. However, in determining whether your status has been abandoned, any length of absence from the United States may be considered, even if less than 1 year
Remain outside of the United States for more than 2 years after issuance of a reentry permit without obtaining a returning resident visa. However, in determining whether your status has been abandoned any length of absence from the United States may be considered, even if less than 1 year
Fail to file income tax returns while living outside of the United States for any period
Declare yourself a “nonimmigrant” on your tax returns
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