Thank you very much for your help. Here are some more sentences I'm not sure of.
1)Each South-American family owned more than twenty slaves on average.
2)Slaves were mostly needed to work on plantations. Their task was to grow and harvest sugar cane. In those profit-making enterprises a system of terrible punishments and rewards was developed (devised, adopted are possible?) in order to increase the productivity and to avoid rebellions.
3)The abolitionists gathered in (into?) the Republican Party.
4)Winston modifies historical records to match the Party’s version of the history (better: of the past).
5)Orwell speaks about this future state (tells about?), which is constantly in war in order to satirize present society.
6)Citizens don’t know anything about who actually are the members of the Party and what is their purpose.
7)The question of slavery was most urgent (when can I use the superlative without the article?).
8) Through the window pane of his apartment he can see some posters. One of these represents(better:portrays, depicts?) Big Brother, a black-moustached man that/who is looking down at the captions “Big brother is watching you”.