4 ideas
Winston Smith, a clerk at the Ministry of Truth who rewrites old newspaper records, starts a forbidden diary and a secret affair with Julia, then trusts O'Brien, an Inner Party member, as a fellow conspirator. In Room 101, faced with rats, his worst fear, Winston begs for them to be set on Julia instead, and the book ends with him sincerely loving Big Brother.
Newspeak is a language built to lose words each year. The Party deletes or narrows words like 'free' so that heretical thoughts cannot be put into words and therefore cannot be thought. Control of the available vocabulary sets the outer limit of what a population can think.
Doublethink means holding two contradictory beliefs at once and accepting both. The believer deliberately forgets whatever is inconvenient, then forgets the act of forgetting. It lets people follow a regime that reverses its claims overnight, like which country Oceania is at war with, without any felt dishonesty, because loyalty depends on switching off one's own memory and logic.
The Party slogan is: 'Who controls the past controls the future; who controls the present controls the past.' Records are constantly altered to match current policy, so no outside standard exists to test the regime's claims against. When there are no reliable records, reality becomes whatever those in power say it is, and the final demand is to accept that 2 + 2 = 5.