Sentence 1 states an assumption that is actually not present in the argument, and sentence 2 correctly states an assumption but provides no discussion of its implications.
It is rather like a debate. These professors not only give you an accurate assessment of your essays, but will also give you valuable insights as to where exactly you can improve your writing skills.
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Instead, try to provide logical arguments and specific examples. Once you know what points you want to prove and what examples you will use to prove that point, writing the essay will be very easy.
What claims are made? Try introducing the opposing viewpoint, showing that you recognize that in fact some people do not support your position.