I am writing to you to complain about... 我写这封信是为了投诉..."本句型是典型的投诉信件的开头。complain about sth.是固定搭配，表示“抱怨某事物”，about不能替换为其他介词。句型拓展1. I am writing to express my dissatisfaction with your service.
我写这封信是想表达我对于你们服务的不满。
2. I very much regret to have to inform you your service is very dissatisfactory.很抱歉地告诉你，你们的服务非常令人不满。 长按识别二维码可关注该微信公众平台