"Whoever controls the past, controls the future: Who controls the present, controls the past."
This quote has a strong significance in 1984. It pretty much lets us, the readers, know how the Party has so much power and control. The Party figured out how to control the past, somehow, by changing history. If something changes in the world, the party prints it out in the newspapers and such as if it has always been like that. During Hate Week, the target of their hate changed right in the middle of a rally, and no one even blinked an eye. Now everyone knew that they target had changed, and the changed target had been their target all along. Winston knows that this goes on, because it is his job in the Ministry of Truth to change things in articles of newspapers and magazines that are now "wrong."
The quote perfectly explains that if you can get control of the past, then you also have control of the future. And the way to get control of the past, is to control the present. It is saying that if you get control of the present and everything in it, you have the power to change things that happened in the past and plan out what will happen in the future. Pretty much, if you have power now, you have had power always and you will have power always...
Unless someone tries to stop you. Which is what Winston and Julia tried to do. Sadly, it didn't work. Everyone who they thought they could trust (O'brien and Mr. Charrington) revealed themselves to be against them and turned them in. It goes to show you that in their world, you really can not trust anyone. Your closest friend could turn against you. That is what the government has done with their control of the present, and in order to keep that control in the future, they make sure that everything goes as they like it. They brainwash everyone to turn anyone, including parent, children, brothers, sisters, best friends, lovers, etc. The government has complete control over everyone, and the worst part is is that most people don't even see it.
