In 1984, what is reality control and how does Big Brother apply it to influence a large number of people?
"Who controls the present, controls the past. Who controls the past, controls the future." This is at the core of your question. We base a good deal of our sense of who we are in the present on who we perceive we were in the past. Actually most of us have no idea of who we (our culture) were in the past; we know what we were taught, what we read, what we find on the internet, and, perhaps most importantly, what our politicans/the media tell us it was.
If the past actually existed, we would be able to visit it and know just what happened and why it happened. Since we cannot, the past is almost infinitely malleable, depending on what people want us to think about it. (As a note, the same is actually true of the present ... the Democrats now tell us that every economic evil in our world was caused by the Republicans ... but could this be true? Did some of them start with Clinton? What about the Democrat controlled Congress of the past two years? Who among us really knows? I have tried to find out, and I can say that I can't figure it out.)
So "reality control" really boils down to information control. And Big Brother and minitru are experts at this!