In 1984, Winston says that he understands the how, but not the why behind the Party. What is the why portion and what page can i find it on?
The exact page number is going to depend on which version of the text that you are reading. It's from part 1, and in chapter 7. The quote is 5 paragraphs from the end of the chapter.
The full quote goes like this:
"The past not only changed, but changed continuously. What most afflicted him with the sense of nightmare was that he had never clearly understood why the huge imposture was undertaken. The immediate advantages of falsifying the past were obvious, but the ultimate motive was mysterious. He took up his pen again and wrote:
I understand HOW: I do not understand WHY."
The reason that Winston understands how the party changes the past is because it is his job. Winston works at the Ministry of Truth, and his job is to change historical records to whatever the party presently needs them to say. Winston absolutely understands how the changes occur. He does them. He just doesn't understand why. He doesn't see a point to it, especially when the historical events are old and obscure events that people barely remember any longer. Winston will eventually learn why the Party wants the past to say whatever currently benefits them. In the Party's own words:
'"Who controls the past controls the future: who controls the present controls the past,"' repeated Winston obediently.
Throughout the novel, Winston secretly hates the Party and its oppressive regime. Being an Outer Party member and an employee at the Ministry of Truth, Winston is aware that he manipulates the past by altering historical documents on an everyday basis. Winston Smith is also an intelligent man who understands the inner workings of the Party. He realizes that Big Brother uses propaganda, fear, and economic disparity to control the population. However, Winston questions why the Party goes to great lengths to create such an oppressive society. The answer to Wilson's question is presented in Part Three of the novel. While Winston Smith is being tortured by O'Brien, O'Brien asks Winston to answer his own question of "Why?" When Winston gives an incorrect response by saying that the Party seeks complete rule for "the good of the majority," O'Brien responds by saying,
"It is this. The Party seeks power entirely for its own sake. We are not interested in the good of others; we are interested solely in power. Not wealth or luxury or long life or happiness: only power, pure power" (Orwell, 332).
Essentially, the ruling elite of Oceania are only interested in complete power. They wish to rule forever and are willing to do anything to make sure their reign is permanent.
No problem :)