Is it typical that the border protection people take one's green card after an absence or do they warn u first?

I have to go back to my country of origin (Czech Republic) for 7-8 months. I'm worried that I won't be allowed to come back. Can this happen or will I be able to get away with a warning first? I'm not willing to live in the Czech Republic afterwards. I only need to go there temporarily for personal reasons. Do the border protection people warn you first? I have read in some post that they don't take your green card if your absence doesn't exceed 1 year and if you don't do this multiple times. If I have an American bank account, evidence of tax-returns, so is it likely that they will take my green card? What else do I need to prove I want to live here permanently?