Once you receive a green card, you must meet a few conditions if you want to keep it for life. For one thing, you must not violate certain criminal or immigration laws -- including one law that requires you to advise the immigration authorities within ten days if you change addresses. For another, you must not abandon the United States as your permanent residence.
If You Violate the Law
The most common way that people lose their right to a green card is by committing a crime. Unlike what is commonly believed, it doesn't have to be a major crime or a felony. For example, a person can be removed from the United States (deported) for helping someone enter the United States illegally, for committing domestic violence, for possessing even a small amount of drugs, or for any crime that's considered morally wrong (such as fraud, theft, a crime with the intent of doing great bodily harm, or a sex offense). Some of these crimes are misdemeanors that may not be punishable with time in jail.
However, there is no set list that tells you which crimes make you removable. If you are arrested for anything at all, consult not only a criminal lawyer, but also an immigration lawyer to find out whether and how you can avoid removal (deportation). Very few criminal lawyers understand the immigration laws -- and many of them encourage you to plead guilty to something as a way of avoiding jail time, not realizing that your guilty plea may get you deported. See Nolo's Lawyer Directory for detailed profiles of attorneys in your local area.
In addition, a person can be removed for certain violations that don't fall under the criminal laws. For example, if U.S. Citizenship and Immigration Services (USCIS) discovers that you got your green card through a fraudulent (sham) marriage, or any other type of fraud, you can be deported.
An immigrant can even be removed for failing to advise USCIS of a change of address within ten days of moving. In the past, USCIS almost never did anything about this. However, with increased security concerns, USCIS has begun using this rule against people it wishes to remove from the United States. You can now use USCIS's online service to notify it of your change of address.
If You Live Outside the U.S.
Many people wrongly believe that to keep your green card all you need to do is enter the U.S. at least once a year. The fact is that if you ever leave the U.S. with the intention of making some other country your permanent home, you give up your U.S. residency when you go. The border officials will look at your behavior for signals that your real place of residence is not the United States.
As a general rule, if you have a green card and leave the United States for more than one year, you may have difficulty reentering the country. That is because the U.S. government feels that an absence of longer than one year indicates a possible abandonment of U.S. residence. Even if you do return before one year, you may run into trouble. To avoid a full-scale inspection, return within six months.
On the other hand, remaining outside the U.S. for more than one year does not mean you automatically lose your green card. If your absence was intended from the start to be only temporary -- for example, you left for vacation, but had a head injury and forget who you were for a year -- you may be able to argue to keep your permanent resident status. However, you may no longer use your green card as a U.S. entry document. You must have what is known as a reentry permit, or you must apply at a U.S. consulate for a special immigrant visa as a returning resident.
The Commuter Exception
Green card holders who commute to work in the U.S. from Canada or Mexico on a daily or seasonal basis may keep their cards even while actually living outside the country. USCIS will grant you commuter status if you advise them of your intention to live on the other side of the U.S. border.
Returning Resident Visas
If you stay outside the United States for more than one year and do not get a reentry permit (described below) before leaving, you must apply at a U.S. consulate abroad for a special immigrant visa as a returning resident. You must convince the consular officer that your absence was temporary and you never planned to abandon your U.S. residence.
You will have to show evidence that you were kept away longer than one year due to unforeseen circumstances. Such evidence might be a letter from a doctor showing that you or a family member had a medical problem.
If you hold a green card and know in advance that you must be outside the United States for more than one year, it's worth applying to USCIS for a reentry permit. This lets you to stay away for up to two years.
You should send in your application before leaving, and wait until you've been called in for your biometrics (fingerprinting) appointment. Use Form I-131, available on the USCIS website. Your reentry permit will serve as an entry document when you are ready to return.
Reentry permits cannot be renewed and can be applied for only inside the United States. If you want to stay away for more than two years, you must return briefly and apply for another reentry permit.
File for Citizenship to Avoid These Problems
You can lower the chances of losing your residence in the United States by applying for citizenship, as soon as you are eligible. The waiting time for eligibility is usually five years after you get a green card, but there are exceptions: For example, the wait essentially drops to four years if you received asylum (because your first year as an asylee counts), and to three years if, at the time you got your green card, you were married to a U.S. citizen and you're still married and living together.
For more information about qualifying for citizenship, see Nolo's article Applying for U.S. Citizenship and Nolo's book Becoming a U.S. Citizen: A Guide to the Law, Exam & Interview, by Ilona Bray.