Re: letter to professor
The main thing wrong with your letter is that you have not done what you were asked to do - Write a letter to your professor outlining why you are not satisfied with their course.
In your letter: - write the reasons why you are not satisfied with the course
In your letter, you have explained why you do not wish to take a course, not why you are not satisfied with it. The examiners will be expecting you to say that the course was not what it claimed to be, that the level was to high/low, that too much/little material was covered, or some other reason for your dissatisfaction.
It is essential in IELTS that you do the prescribed task. If you do not, you will not gain a good mark, even if your English has no mistakes at all.
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