Lawful permanent residents of the United States may travel abroad freely for less than a year. But traveling abroad might be difficult if you hold an expired Green Card. Even if your Green Card is valid you will be permitted to enter into the country only after inspection and the US immigration officers will question you in order to make sure that you had not left the country with an intention of giving up your lawful status in the United States. US Green Cards are issued to foreign nationals who intend to make the country their permanent home and if such Green Card holders plan to reside permanently in a foreign country, their Green Cards may be revoked and they may not be allowed to re-enter the country. You may be at the risk of losing your Green Card if you travel outside the country frequently. US immigration officers at the US airport will also ask you about the purpose of your frequent travel.
However, you may not be allowed to re-enter the country with your valid Green Card, if you had lived in a foreign country for a year or more. And you will have to obtain a re-entry permit to get back to the country, prior to your departure, if you plan to remain abroad for more than 365 days. But if you had to remain abroad for long time due to a genuine reason, you may explain your situation to the US immigration officer and provide evidence to prove that you had not planned to remain outside the country but you had to remain due to a valid reason. If your family member had fallen sick all of a sudden and if you had to stay in the foreign country due to that reason for a long period of time, you may submit copies of medical records to show that you had to remain with the family member who was in need of your help. However, your continuous absence from the country disrupts your continuous residence and the US immigration officers may doubt that you have plans to live in a foreign country and abandon your permanent residence status in America.
In order to prove that you had to remain abroad for a genuine reason, you may be required to provide documentary evidence to show that you have social and family ties in the United States. Hence, it is wise to always carry with you certain documents that will help you to prove that you intend to reside in the United States permanently. You may provide details about your property in America, the vehicle that you own in the United States, your American bank accounts and your US driver's license, to demonstrate that you need to return to the country. Similarly, the tax returns that you had filed and your employment in the United States, may also help you to establish that you had do not have an idea to relinquish your permanent resident status in America. Apart from that, you may present copies of your family member's medical reports to show that you had to stay with that relative, as he needed your help. A US immigration officer at the US airport may expect the above mentioned documents and evidence to grant you permission to re-enter the country.