Suppose for a moment that Mimi has a paper due, but that she doesn't agree at all with the topic. Instead of posing a well-thought out argument supporting her position, Mimi writes a paper which boils down to "My religion says this is a bad thing so that means it is a bad thing." She receives her paper back and is surprised to see a grade that is the exact opposite of what she believes she deserves. Joe, on the other hand, writes a response that doesn't actually address the initial question.
One way to avoid this is to outline your argument before you start to write. If you start far enough in advance, if you find out that you don't understand the topic, you have time to discuss the problem with your professor. By outlining the argument, you can make sure you answer all parts of an essay question, fully back up your argument, and avoid some of the many invalid argument forms.
Tip: Make sure you have at least three reasons for your position.