In 1984, does the reader understand why the party does what it does?

In 1984 the reader easily understands why the Party acts as it does.

The motivations for the Party's actions are not entirely clear from page one, but more astute readers may be able to decern the psychological ramifications of certain Party doctrines. Concepts introduced early on in 1984, such as the thought police and the constant monitoring of all Party members, are easily seen to be instruments the Party uses to control its constituents. It can be extrapolated that even if Party members are not always monitored, they will surely be frightened into good behavior at all times under threat of being taken to the Ministry of Love. 

However, later on in this novel, there is a much more definite outline of the Party's motives and goals. In chapter nine of part two, the reader is able to see the word for word content held within Goldstein's book. Entitled The Theory and Practice of Oligarchical Collectivism the reader is given two of the book's chapters, each speaking in-depth about the Party's doctrine and goals. As Winston reads along we see chapter one titled "Ignorance is Strength" and chapter three "War is Peace"; the names of said chapters obviously correspond to parts of the Party's full slogan as seen below. 

War is peace.

Freedom is slavery.

Ignorance is strength.

Further on in 1984 O'Brien also answers many of Winston's questions about the Party and what it truly seeks to achieve. 

As well, some editions of 1984 have an appendix called "The Principles of Newspeak" wherein there is greater clarity given to why Newspeak is needed. It may seem pointless to read an appendix, and I know some students chose to skip over the portions we are given of Goldstein's book, but they do shine quite a bit of light on the Party's plans and intentions -- I strongly recommend that students try to muddle through at least Goldstein's book.