I really have problems in correcting the grammar and choosing the proper synonyms in these sentences. I selected the six most difficult ones.
1) Their task was to grow and harvest sugar cane. In those profit-making enterprises a system of terrible punishments and rewards was developed (better: devised, adopted are possible?) in order to increase the productivity and to avoid rebellions.
2)The abolitionists gathered in (into, ?) the Republican Party.
3)Orwell speaks about this future state (tells about?), which is constantly in war in order to satirize present society.
4)Citizens don’t know anything about who actually are the members of the Party and what is their purpose. (anything about the Party members and their objectives.)
5)The question of slavery was most urgent (when can I use the superlative without the article?).
6) 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.
2 answers
4) you might reword to: who the members ac tually are of the Party and what their purpose is.
5) this is a good example, with "most" or "very" etc.
The things in parantheses are all possible; some are better than others.
Sra
In #3, don't use "speaks" or "tells" -- use "writes" since it's his book you're referring to, right? Or did you hear him speak?