A good question, and one that I have often wondered about as well.
My first thought is to probably shorten or lengthen time to fit movie length format.
Another may be to make the story more "interesting", in the opinion or eyes of the director and/or screenplay person.
(I personally prefer a movie follow the original story...)

Grendel's retreat into wood is more or less A resolution of the External Conflict that was in the form of Grendel. The external conflict against Beowulf was to finish off the monster called Grendel. Since, the Grendel is defeated by Beowulf in this part and forced to run away, it is the resolution of the conflict.
Resolution: A resolution is the part of the story's plot line in which the problem of the story is resolved or worked out. This occurs after the falling action and is typically where the story ends.
External Conflict: struggle between a literary or dramatic character and an outside force such as nature or another character, which drives the dramatic action of the plot.
Internal Conflict: psychological struggle within the mind of a literary or dramatic character, the resolution of which creates the plot's suspense

Source(s):

bookswriters.com

I read a book in high school about a man who kidnaps a girl who then has to live in his basement.

If you do not find the name of your book, perhaps this book (The Collector) will fulfill your needs.

Should I watch Lord of the Rings and The Hobbit before playing Middle-Earth: Shadow of Mordor?

Shadow of Mordor is coming out this month and it's in the same world as Lord of the Rings. Do you think I'll need to watch any of the movies to understand what's going on in that game? I've never seen any of the movies and I really have no idea about that universe, but I don't want to watch all of the movies just to understand a video game.

First, learn to write proper English. "Your" and "You're" are not the same thing.

Then - just sit down and get on with it. There is no "right" way of writing a novel, and few "wrong" ways. You DO have to put in the effort, consistently over a long period of time.

And remember:you can write a novel or have a life - but not both.