I don't understand the change of address form. what am I supposed to do with it. I have green card. but I must tell us government where I live? or if I move? what if I live with friends for short time. must I also fill out form? we never have to do this at home where I come from.

Green card holders are required to submit their change of address on form AR-11. You could get in trouble for not doing it. I don't think the requirement is enforced that much, but still, the law is the law. The form should not take you more than 5 minutes to complete and fill out. This is not Russia!