很多英语语法书籍和语法教材在讲到关系代词that的省略用法时,通常仅限于that充当定语从句宾语时的省略。英语学习者在查阅有关that作定语从句主语的用法时,往往会看到“不能省略”或“一般不能省略”的规则。然而,任何语法书都不可能包罗语言万象。譬如,在《21世纪大学英语》读写教程第四册第五课的A课文中就有这样一个句子: Many English grammar books and grammar textbooks are usually limited to the omission of the attributive clause object when it comes to the omitted usage of the relative pronoun that. When an English learner looks up the use of that as the subject of the attributive clause, he or she often sees the rule of “cannot be omitted” or “cannot be omitted.” However, any grammar book cannot include all languages. For example, in the “A 21st Century College English” reading and writing tutorial, Lesson 4, Lesson 5, Lesson A, there is such a sentence: