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1、that在定语从句中做主语、表语或宾语，作宾语时可省略。

2、必须用that的情况：先行词有人又有物先行词有形容词最高级修饰先行词是不定代词先行词有不定代词修饰以who，which开头的问句先行词有the very, the same ,the last等词修饰先行词有序数词，数词修饰时。

3、关系词在定语从句中做表语时不能用that的情况：介词+关系词非限定性定语从句 举例： Is this the book that you borrowed in the library? 这是你在图书馆借的那本书吗？ Who that break the window should be punished． 谁打碎了窗户都要受到惩罚． All that is needed is a supply of oil. 所需的只是供油问题。

4、 Finally, the thief handed everything that he had stolen to the police. 那贼最终把偷的全部东西交给了警察。

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