If it seems unclear, ask the professor for help. It is impossible to write a coherent paper without understanding the question. Check the Course Reserves. Some professors use the reserve room to display examples of good writing. These models will help a student understand the expectations of the professor.
Students forget to use verbs from time to time.
It seems a basic grammatical mistake, but some students believe that clauses can stand on their own as a complete thought. Read your paper out loud. If a student is unsure of the structure of the paper, or whether it makes sense, reading the paper out loud proves to be the best exercise for making sense out of it. Spell-check your paper. There is absolutely no excuse for misspelled words.