paper
［误］ Please give me two letter papers.
［正］ Please give me two sheets of letter paper.
［析］ paper作为纸讲为不可数名词。
［误］ Each student must write paper on what he learned.
［正］ Each student must write a paper on what he learned.
［析］ 这里的paper作为论文讲，是可数名词。paper的这种用法还可以用于证件、报纸、考卷等。
parent
［误］ My parents and I are both interested in football.
［正］ My parents and I are all interested in football.
［析］ parents即为父母、双亲，指两个人，加上自己为三个人，所以只能用all而不能用both.
