It's not as simple as you think
my research on the internet shows (
http://antaoandchuang.com/i-keeping.html):
--------------------------------------------------------------------------------
This document describes certain precautions for U.S. permanent residents ("greencard" holders) to take in order to maximize the chances of being able to maintain permanent residency status, i.e., "keeping your greencard." Please note that the purpose of this document is help law-abiding persons avoid common pitfalls that could cause them to inadvertently lose their greencard. It is not intended for anyone who should not legally be able to keep their greencard.
--------------------------------------------------------------------------------
GENERAL RULE
It is very important to realize that U.S. Immigration law requires that a greencard holder must NEVER abandon the INTENTION of continuing to reside permanently in the United States. Once, a greencard holder abandons that intention, e.g., by intending to reside permanently in some other country, that person loses the right to keep their greencard.
This means that ANYTIME a greencard holder leaves the U.S., that person is subject to being accused by the Immigration and Naturalization Service (I.N.S.) of having abandoned the intention of living in the U.S., and is subject to having the greencard taken away--on the spot. Therefore, greencard holders must always take certain PRECAUTIONS in order to be able to prove to the I.N.S. that they have never abandoned the intention of living in the U.S. permanently. However, simply returning to the U.S. once a year for several weeks to use the Green card is NOT enough!!! Many green card holders have lost their greencards--even though they returned to the U.S. once a year.
--------------------------------------------------------------------------------
PRECAUTIONS
Consequently, it is very important that other precautions, such as the following, be taken:
1. OBTAIN A REENTRY PERMIT
If you will be leaving the U.S. for more than one year, but less than two years, it is advisable to obtain a reentry permit prior to leaving the U.S.
Note that if you continue to request a number of reentry permits (one after the other) the I.N.S. is likely to deny your application.
2. FILE TAX RETURNS
ALWAYS FILE A U.S. RESIDENT TAX RETURN, AS WELL AS ANY OTHER APPLICABLE STATE, CITY, OR LOCAL TAXES.
Be sure to file resident tax returns, e.g., 1040, and not a non-resident tax return!
Note that this does not necessarily mean that you must actually pay U.S. income taxes, it only means that you must file a resident tax return and declare your worldwide income on that return, even if most of this income is exempt from taxation. Consult a tax advisor where applicable.
Keep copies of all tax returns you have filed as a resident, and BRING THESE COPIES WITH YOU WHEN ENTERING THE UNITED STATES.
3. MAINTAIN A U.S. ADDRESS:
Maintain a U.S. Address, even if the address is the home of a friend or relative.
If possible, do not use "in care of" for your address.
DO NOT have a U.S. address which is a resort or hotel.
4. MAINTAIN U.S. BANK ACCOUNTS
You should leave open and continue to use U.S. Bank accounts.
For example, some employers, when assigning an employee overseas, will continue to pay the employee in U.S. dollars, depositing the amount directly into the employee's U.S. account.
5. MAINTAIN U.S. DRIVER'S LICENSE:
You should continue to RENEW your U.S. Driver's license.
Be sure that the address on your license is the same as that recorded on any immigration documents.
Carry your driver's license when entering the U.S.
6. MAINTAIN U.S. CREDIT CARD ACCOUNTS
7. OWNERSHIP OF U.S. PROPERTY
If possible, continue ownership of U.S. property, e.g., houses, condominiums, businesses, automobiles, etc.
For example, a person assigned abroad may want to rent, rather than sell, his or her U.S. residence.
8. DOCUMENT REASONS FOR LONG STAYS ABROAD
EMPLOYMENT CONTRACTS: Before leaving the U.S. on long assignments abroad, it is advisable to obtain a written employment contract, or letter, from your employer.
The contract or letter should specify the terms and length of employment.
If the employment will lead to a transfer back to the U.S., or to a U.S. based affiliate of the foreign employer, the contract or statement should include this fact.
9. SOCIAL SECURITY NUMBER
Be sure to maintain your social security card, and carry it with you on your return to the U.S.
If you have not yet done so, be sure to apply for the type of social security card which permits you to work in the U.S.
10. SELECTIVE SERVICE
For persons of applicable age, be sure to register with selective service.
11. KEEP YOUR GREENCARD VALID:
Be sure to renew your greencard when it expires.
Children who reach the age of fourteen (14) must file an application to replace their green card unless the prior card will expire before they reach age 16.
12. THINGS TO AVOID:
DO NOT return to the U.S. using any form of non-immigrant visa. For example, if you stay outside the U.S. for longer than one year, without having obtained a reentry permit, do NOT return to the U.S. on a tourist visa; instead, apply to the U.S. consulate for a Special Immigrant Visa.
DO NOT arrive via a chartered air carrier where nearly all passengers are nonimmigrant.
DO NOT return to the U.S. with a spouse and or children who are neither citizens nor green card holders, especially if they will be in the U.S. only a short time.
DO NOT arrive at a port of entry functioning as a gateway to a resort area.
DO NOT enter the U.S. on a round-trip ticket which terminates outside the U.S.
---------------------
NOTE: That even if you take all of the above precautions, there is NO GUARANTEE that the I.N.S. will not exclude you from the U.S. for other reasons, e.g., such as for committing a crime involving moral turpitude. The above information is intended solely as general reference information. It is not legal advice. Consultation with an immigration attorney is recommended. In addition, please note that Immigration Laws frequently change.
--------------------------------------------------------------------------------
