What did the belligerents expect at the beginning of wwi, and why did the course of the war turn out to be so different from their expectations? how did wwi affect the belligerents' governmental and political institutions, economic affairs, and social life?

At the beginning of WWI, the belligerents expected to have a brief, fast war, that could be concluded in weeks, at maximum months. They do not expected the war to carry on for years, with all the consequences that this simple miscalculation had on the life of everyone affected by war, both in Europe and in the other continents.

The course of the war turn out to be so different from their expectations because of many factors: the most important perhaps was the fact that the way countries and armies did war was drastically different, so instead of a fast war what happened was a trance war, very slow and very deadly for the civils.  
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WWI affected the belligerents' governmental and political institutions, economic affairs, and social life by subtracting a lot of resources in every sector, and by costing a lot of resources in terms of money and life loss. The governments wanted to prove their supremacy but ended up being almost in bankruptcy.