How Will Absence From United States Affect My Green Card?

My father was taken very ill and he is not able to travel back to the U.S. I need to go be with him for some time, I think for more than one year. How will this absence affect my green card? I need to make a decision, can you help me?
If you are a green card holder, and plan to be away for up to two years, you should definitely file for a Reentry Permit. This travel document will allow you to be absent from the U.S. without putting you permanent residence in jeopardy. Remember though, that the time spent away from the U.S. will not be counted towards qualifying you for citizenship.