that是什么词
that的词义很简单，用法却非常复杂，但是通过仔细分析归纳不难发现其用法可分为两大类：
that做限定词、副词、指示代词时，必然表示「那」（那个、那样、那种、那么），此时that单独使用，或后接其他词，组成词组;
that做引导词时，必然后接一个句子（完整或不完整），组成某种从句或感叹句。
1、that做限定词、副词、指示代词
限定词，表示「那个」，用于名词词组
Not that one!
副词，表示「那么」，修饰形容词、副词、介宾短语等
You’re not that stupid形容词.
指示代词
表示「那个」
Most businesses seek to make a profit--that is (to say), they aim to achieve income that is more than the costs of operating the business.
表示「那样」
– He’s a fussy guy.
– He is that.
2、that做引导词
引导定语从句
关系代词：先行词 + (that关系代词 + 不完整句)定语从句，关系代词that表示「那个」「那样」，在定语从句中做主语、宾语、补语等。
Then there’s the kind of popularity that appears in adolescence: status born of power and even dishonorable behavior.
关系副词：替代when、where、why、for which、in which等，常与day、place、reason、way等表示时间、地点、原因、方式的词连用。
The exact time and place that=when/where crops were first cultivated successfully is uncertain.
引导名词性从句
连词：{that连词引导词 + 完整句子}名词性从句，无具体词义。
They found that more on-demand entertainment viewing on tablets instead of TVs and desktop computers could cut energy consumption by 44%.
引导状语从句
连词：[that连词引导词 + 完整句子]状语从句，引导状语从句的that无具体词义，一般与其他词组合成provided that、given that、now that、in case that、in order that、in the event that、such…that…、so that…、so…that…等，整体视为引导词，引导条件、目的或结果状语从句。
An actor and actress stood in front of the scene so that they looked as if they were at the water’s edge on an island.
引导感叹句
连词：{That连词引导词 + 完整句子}感叹句，无具体词义，往往用虚拟语气，表示震惊或愿望。
Oh that I could go back to my childhood!
3、引导词that的省略
引导词that引导感叹句不可省略，引导定语从句、名词性从句、状语从句均可能出现省略。
引导定语从句
关系代词在限制性定语从句中做宾语时可省略。
It also doesn’t require a new type of technology (that) people aren’t already familiar with.
关系代词在限制性定语从句中做表语时可省略。
He still talks like the man (that) he was ten years ago.
关系代词在限制性定语从句中做主语，且从句内部是there be结构时可省略。
This is the fastest train (that) there has ever been.
定语从句符合「关系代词 be X」的结构，可省略「关系代词 be」，变为X做后置定语/同位语。X可能是非谓语动词，也可能是名词、形容词、介宾短语等。
People (who/that are) motivated by pleasure seem to suffer particularly badly.
上述that代替关系副词when/ where/ why的情况，往往可省略that。
the reason (that) I phoned
引导名词性从句
宾语从句的引导词that往往可以省略。
Teach your teenager (that) the family phone is for the whole family.
非正式文体中，名词词性从句的引导词that如果省略句意仍然清晰则可以省略。
It’s great (that) you’ve come in time.
