that可以引导名词性从句、定语从句，其具体用法如下：
一、that引导名词性从句时（即主语、宾语、表语、同位语从句），在从句中无具体含义，只起连接作用，而且在从句中也不作任何句子成分。它所引导的从句的句子结构和意义是完整的，而且从句一般是陈述一个事实。例如： ①That she was able to come made us very happy．她能来使我们很高兴。 ②The news that our team has won the match is true．我们队赢了这场比赛的消息是真的。 ③The reason he didn't come was that he was ill．他没来的原因是他病了。
当that引导的主语从句或宾语从句太长时，常用it作形式主语或宾语而把that从句放在后面。如： ①It is well—known that the earth is round．众所周知，地球是圆的。 ②We find it necessary that we practise spoken English every day．我们发现每天练习英语口语很有必要。
that引导的主语从句，表语从句以及同位语中，that不能省略。
that引导的宾语从句，一般可以省去that，但在下面情况中不能省略。
1.在suggest, order等表示命令、请求、建议等动词后的宾语从句中，不能省。 2.由and或but所连接的两个引导的宾语从句时，第一个从句中的that可以省，第二个从句中的that不能省。 3.that引导的宾语从句，如果从句中还有其他状语从句，引导宾语从句的that不能省略。 Tell him that if he is at home, I’ll call to see him.
二、that引导定语从句要注意以下几点：
1．定语从句中缺主语，修饰人时用who ／that，修饰物时用which ／that引导。如： Is this the factory which ／that makes TV sets？ The man who ／that visited our school is from Australia．
2．从句中缺宾语，修饰人时用 who ／whom ／that或省略引导词；修饰物时用which ／that或省略引导词。如： Gone are the days which ／that we spent together in the village． He is the man who ／whom ／that I think to be worthy of our praise．
3、用that不用which的七种情况： ①先行词前有形容词最高级或序数词修饰时，用 that不用which引导。如： This is the best place that I have ever visited． ②先行词为little，much，all，none，few，（the）one， something，everything，nothing，anything等不定代词时，用that引导。如： There is nothing in the world that can frighten him． ③先行词既有指人的名词又有指物的名词时，用 that引导。如： He told us about the people and the cities that he had visited． ④先行词前有any，all，no，few，every，some，little， much，（the）very，（the）only，（the）last等词修饰时，用 that引导。如： This is the very coat that I need． ⑤先行词为which时，为了避免重复，用that引导。如： Which is the book that you bought yesterday？ ⑥先行词在从句中作表语时，常用that引导。如： Shanghai is no longer the city that it used to be in the 1930's． ⑦当主句是There be．．．句型时，用that引导。 There are two tickets for the film that are for you two．
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