Those who control the present, dictate the past, and can control the future.
This phrase is pretty self-explanatory. Say, after a major revolution or war, a new leadership comes into play. They are in charge of writing what has happened over the course of their rise to power. Thus, they can create whatever image of the past that they wish. Editing and ommission come into play, and it is no wonder that history itself hasn't collapsed.
In 1984, Winston is a worker at the Ministry of Truth. Ironically enough, his job is to create and edit propaganda. To rewrite the past as to keep people loyal in the present. Now, this seems to be a little far-fetched, don't you think? But we are guilty of this ourselves today. Our American textbooks may say that we had a nice Thanksgiving meal with the Native Americans, but where do we read the part where we come to slaughter them the next day?
Now, not only does the ruling party dictate the past and present, but they also control the future. Every piece of history that is taught in school gives us a date, place, and how it happened. But do they ever tell us why? No, instead students have facts crammed down their throats without knowing the slightest bit of legitimately useful information. If we don't know why, we can't prevent it from happening again. Who's to say we won't fall victim to another war? Another wave of slavery? Or even a second Holocaust?
Going back to the novel, 1984 is a perfect example of leaders controlling the future. Because they throw all pieces of information that could pose a threat to them if in the wrong hands into the incinerator, they eliminate all chances of a revolt or revolution. They control the present, so they rewrite the past, and therefore dictate the future.
So all in all, is this warning from Orwell? His writing seems to forecast to us need for reformation. A sudden change for the better.
We need to grab ahold of the present, so we can fix our past, and make way for a better future.