As far as I can remember, the protagonist of the story was working in a ministry which was basically rewriting the history. What was their reason? They wanted make people that the everything that the Big Brother said was true. He was the absolute truth. Moreover, the party didn't allow people to write, so that they won't have an evidence that will support the Big Brother is not true, or things were much better in the past. There is this quote form the book "Who controls the past controls the future. Who controls the present controls the past." It clearly shows that for the party to have control over the society, they should be controlling the past. That's why they are changing the past. You should also consider these questions " How can we understand our present or glimpse our future if we cannot understand our past? How can we know who we are if we don't know who we were? " Also think about how the party records/ watches every action of its citizens . And then there are thoughtcrimes, to stop a person from thinking different, or holding an opposing opinion. What they are trying to do is to eradicate ideas/opinions/ differences, to scare people and control them. Furthermore, the party was changing the dictionary and making it shorter. Try to come up with supportive ideas from this quote: " But if thought corrupts language, language can also corrupt thought." How much can you think if you knew only limited amount of vocabulary . I remember something like there weren't really pictures of how cities were in the past or generally anything related to past. If you can't read about the past, and if you can't see it from any other art from, how can you come up with an opinion about it ?
The most important way that party was changing past and stopping rebels( like the protagonist himself) was to erase their minds and thoughts, there was a specific name for it but I don't really know since I read the book in my native language. After that process, rebels became people who were obeying the rules of Big Brother.
Maybe you can think about why the party was pushing its citizens to do certain things for example doing sport every single morning?
If you have discussed it in the class you can also mention about the book and the World War II. How part and the propagandas of USSR are similar.
Good luck !