Here are some mistakes that you should avoid in writing task 1 of the GT test:
- The beginning and ending of the letter don't match.
- The main purpose of the letter isn't clear.
- You didn't cover all of the points.
- You wrote too much about one point, and neglected the others.
- You didn't separate your paragraphs clearly.
- You didn't get the 'tone' right (formal or informal).
- The letter doesn't 'flow' well because ideas are badly organised.
Avoid these mistakes, and you are on the way to writing a good letter!