RC improvement

I have been preparing for GMAT since last two months, I have come across the Powerscore Reading comprehension book recently. I felt unlike other material, it clearly points out each and every challenge I was facing in RC section e.g. struggling with the long complex sentences.

My weak points are very well mentioned in the book, however, I am still unable to understand how to improve that part.

For example, the Language Simplification Drill II on page 71-

10. There is little credibility to the claim that even a
politically polarized nation can distinguish between
legitimate calls to action and the voices of messianic
demagoguery. To speak without listening is an exercise
in futility, for no matter how informed or erudite the
views, they are bound to fall on deaf ears. Unfortunately,
the same cannot be said about shouting.

Decoding this sentence is impossible for me no matter how many hours I sit over it (2nd and 3rd sentences).
Though the problems are very well stated in the book, but the solutions are simply mentioned. It is totally upto the reader to break down the sentences and decode the meaning.


Its been a week, I am stuck with basic Practice drills in the book and I am stuck. What are these type of complex sentences called. Where can I find a source for learning to decode such sentences.

Thanks in advance.