Did they tell you that you can lose your green card if you're out of the country for too long? (unless you have a darn good reason why.. such as military service or government work)..
technically, if stay abroad for a year, you have to apply for permit to come back.. after two years.. in most cases, you're screwed
However, just like anything else, it's at the INS officer's discression to determine whether you "maintained permanent residence in the US during that time"