Ram Pradad Ji,
If you don not maintain residence in the United States without proper authorization, you are considered to have abandoned residence and will forfeit your Green Card.
If you believe that your absence from the U.S is necessary and you have what the INS calls "good cause" you can apply for a "Re- Entry Pemit" which essentially allows you to leave the country for a period of more than a year. Generally, the Re-entry is provided to people who can show that they are in school or have long term interests in another country and that their presence is required there..you could try and make a case saying that your parents are elderly and since Nepal does not have managed care systems for the elderly, you have to go back to Nepal to attempt to make alternate living arrangements for them.
Once you apply for your re-entry, you can leave the country (the INS will send you the notice of approval or denial so you don't have to remain in the U.S waiting for it. You can have them mail it to the U.S Embassy in KTM for Pick up there) If approbved, the re-entry looks like a white passport and will be mailed to you.
The INS websit should have the form that you need under the forms and fees section:
The form you need is an I-131 which must be sumbitted to the INS at least 30 days prior to your departure.
Be aware that if you leave the U.S for more than one year, the 5 years of continuous residence that you need for U.S Citizenship will be affected even if you get the re-entry.
Be sure you consult a qualified attorney to determine what the impact will lilkely be on your Citizenship application down the road. If you do not know of a qualified immigration lawyer, check this website for names of qualified immigration law attorneys:
www.aila.org or call 1-800-954-0254 to speak with a representative or send an e-mail to ilrs@aila.org. In the e-mail, please provide your name, location and what your need is for an immigration lawyer.
Best of Luck.
