Practise complaint letters. Use the following web pages. They’re extremely useful!
–COMPLAINT LETTERS 1: here you’ll find many samples of the most common complaint letters. Then go down on the page and you’ll see tens of FILL-IN-THE-GAP exercises. Try them!!
–COMPLAINT LETTERS 2: here there’s a thorough set of exercises of many different kinds, all on complaint letters: word ordering, matching, gap filling… YOU CAN’T MISS THEM!
-Finish with this last exercise.