that是个常见词，可以引导不同种类的从句，用法比较复杂，学生在学习中容易混淆他们的用法。从留预教材出发，本文将详细总结that在从句中的使用，分析比较它们的用法，以帮助学生更好地掌握that从句，让学生在学习其他语法现象时有所借鉴。
一．that 引导名词性从句
名词性从句有四种，分别为主语从句、宾语从句、表语从句和同位语从句。That都可以引导这四种从句，接下来根据文章中已有的句子来分析that在这四种名词性从句中的使用。
1. That引导主语从句
主语从句，顾名思义，在句子中充当主语的句子叫做主语从句。That如果在主语从句中充当主语，那么that就不能省略。
（1）that在主语从句中充当主语，我们先从以下例子出发。
That the environment needs protection is a truth. 环境需要保护是一个真理。
That 在这里引导主语从句，与其他引导词的区别是，that后面的句子属于陈述句而非疑问句。另外，在我们的阅读文章中很少会出现这种that在句首引导主语从句的情况，因为会造成句子头重脚轻，因此，在留预教材中多会出现以下形式的主语从句。
It is not surprising that it is considered one of the biggest ever changes in modern societies.(三级unit 1 reading 2)
It is thought that the world’s population will reach about nine billion in 2050.(三级unit 2 reading 2)
It's no surprise that we tend to believe that a person’s personality is stable.(四级unit 1 reading 2)
以上三个句子的结构是：it be + adj/过去分词/n + that + 句子。这里的it都是形式主语，指代的都是that后面的从句，在理解整个句子的意思是，要把that后面的从句挪到前面，然后翻译，it不翻译。这些句子也是主语从句，只是为了避免头重脚轻，所以才需要用it在句首充当形式主语。
2. That引导宾语从句
有许多动词后面可以跟 that 引导的从句作宾语，如 agree，believe，claim，decide，expect，find，guess，hear，insist，know，learn，realize，suggest，think，tell，wish 等，口语中 that 常可省略。
Not everyone believes it will work.(三级unit 5 reading 2)此句中believe引导的宾语从句就将引导词that省略了。
但在下面情况中不能省略：
，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。
New evidence, however, suggests that this is true.(四级unit 1 reading 2)（suggest表暗示，that可以省略）
He suggested that I (should) stick to my decision.（suggest表建议，that不可省略）
第二，由and 或 but 所连接的两个宾语从句时，个从句中的 that可以省，第二个从句中的 that 不能省。
Many felt that they returned clothes to online stores too often and that this made the whole process slower and more difficult.(三级unit 4 reading 2)
此句中felt后面有两个宾语从句，因此that不可以省略，这个句子中两个that都出现，前者可省，后者不可省。
第三，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。
While many argue that there is nothing wrong with having big ambitions, this worrying trend could lead to people feeling unhappy as more and more are unable to reach their goals.(三级unit 6 reading 2)
此句中while引导的对比状语从句，因此argue后面宾语从句中的that是不可以省略的。
3. That引导表语从句
That引导表语从句，that要放在系动词的后面，有时候可以省略。
Maybe it’s just that I have a romantic notion of handcrafted furniture.(四级unit 2 reading 2)
4. That引导同位语从句
So finding it in the Antarctic was proof that these countries and continents had all been joined together in the past.(三级 unit 8 reading 1)
此句中that便是在引导同位语从句，that后面的内容是在说明proof这个名词的具体内容。以下两个也是同位语从句的典型例子，像fact，idea，belief，conclusion，evidence，information，message，news，opinion，order，proof，proposal，report，rumour，rule，suggestion，truth，view这些名词后面都可以跟同位语从句，最后一个例子中的这个同位语从句在平时的学习中很多学生都会使用，只是大家会忽略这是一个同位语从句，doubt的内容就是that后面的内容。
They benefit from the fact that sound waves travel five times faster through water than air.(四级unit 9 reading 1)
Many ways we compare are financial, but there is no doubt that many feel the pressure from the birth of their first child.(三级unit 6 reading 1)
二、that 引导定语从句
在定语从句中，that只能引导限定性定语从句。并且that在从句中可以充当从句的主语，宾语或者表语，当that充当主语时不可以省略，但是另外两者可以省略。
The job that was once the only focus of their lives becomes something they are no longer involved with.(三级unit 6 reading 2) （that充当主语，不可以省略）
Bats make sounds that echo back to them to give them a better understanding of where things are in their environment.(四级unit 9 reading 1)（that充当主语，不可以省略）
One approach many entrepreneurs take is just to improve access to products that already exist.(三级unit 3 reading 2)这个句子中many entrepreneurs take是一个定语从句修饰approach，但是这里省掉了引导词，可以是that也可以是which，which和that在限定性定语从句中可以互换。但是 that already exist也是定语从句修饰products，这里that做了主语，因此that不可以省略。
在定语从句中有七种情况只能使用that引导而不能使用which，如下：
①先行词前有形容词最或序数词修饰时，用 that 不 用 which 引导。
Selfies are rarely the first photo that some took.(三级unit 1 reading 2)
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。
Research around the world suggests this is not only a Japanese problem but one that exists around the world.(三级unit 1 reading 2)
③ 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。
The 750,000 people who visit it every week can find almost any product that meets their demands. (三级unit 4 reading 1)
④先行词既有指人的名词又有指物的名词时，用 that 引 导。
Humans and animals which exist on earth are equal.
⑤先行词为 which 时，为了避免重复，用 that 引导。
Which is the scenario that you love best？
⑥先行词在从句中作表语时，常用 that 引导。
These risks are not stagnant from the point of launching a product, but are an ever-evolving challenge that needs constant assessment.(四级unit 10 reading 2)
⑦当主句是 There be……句型时，用 that 引导。
Of course, there are techniques that workers use to prevent injuries, but they’re not always entirely successful.(四级unit 2 reading 2)
三、that 引导状语从句
1.that 引导目的状语从句
Watches have been rebranded so that they are no longer simply tools for telling the time.(三级unit 3 reading 1)
这里so that表示因此，所以这是一个目的状语从句。
2. that 引导结果状语从句
Some 3D maps have so much detail that researchers have discovered hidden rooms in buildings.(二级unit 5 reading 2)
The pressure to be prefect is so strong that it leads to many parents lying about how much their baby sleeps at night.(三级unit 6 reading 1)
目的状语从句和结果状语从句所使用的形式都有so和that，但是在前者中so that是需要连用的，后者中so和that之间需要加形容词。另外，结果状语从句也可以使用such +名词+ that的形式。
3. that 引导条件状语从句
条件状语从句主要由 provided that，on condition that, supposing that 等引导。
Provided that the biodiversity is protected, the environment will become better.
4. that 引导原因状语从句
原因状语从句主要由 now that 等引导以及一些“be + 形容词”结构后的从句
Prevention of danger is vital that many states gives it priority.
四、that 用在强调句中
强调句型中，当强调的内容不是人物时，通常会使用that，这里的that通常也没有必要翻译，并且强调巨大额特征就是去掉it be that之后整个句子在逻辑和结构上都是完整的。
That用在强调句中
It’s not just New York’s parks and transportation that have changed, but it’s boroughs have too. (三级unit 5 reading 2)（强调主语）
James Clerk Maxwell first proposed the theoretical and mathematical form of wireless communication in 1864, but it wasn’t until 1888 that Heinrich Rudolf Hertz conclusively proved that wireless communication would work.(四级unit 9 reading 2)（强调时间状语）
五、小结
That从句在留预教材中大多数以宾语从句，定语从句和主语从句的形式出现，这三类从句最多。在理解的时候需要判断that引导哪种从句，从而结合句子其他成分理解句意。这能帮助学生在考试的时候识别句子的类型从而快速判断句子的结构理解句子的中文意思。
