谈谈我自己的领悟把：
英语语法中很重要的一部分是从句.
从句有分为三种：状语从句 定语从句 名词性从句（主语从句、表语从句、宾语从句、同位语从句）.
“关于这些的介绍可以到摆渡知道中找,多的是.”
在由that引导的从句中,可以省略的有：定语从句中作宾语时、宾语从句中.其它的不可以省略,如：定语从句中作主语、同位语从句等.
bu:三、 只用that引导和不用that引导的场合
1． 只用that引导和不用that引导的场合
1） 当先行词既包括人又包括物时：He talked about the men and the books that attracted
him.
2） 指物的先行词被any, every, only, very, all, no等修饰时：These are the very points that interest me./That´s the only watch that I like most.
3） 指物的先行词被序数词或形容词最高级修饰时：The first step that we are to take is very difficult./This is the second card that he gave me.
4）先行词是指物的不定代词all,little,few,much,及everything,anything,nothing等时：There is still much that can be done about it./Have you got everything that you need?
5） 先行词是who时：Who that have seen him does not like him?
2． 不用that的场合如下：
1）非限制性定语从句中 Last night ,I saw a very good film,which was about the Long
March .
2）介词放在关系代词之前时 This is the man from whom I learnt the news
3）句中出现了that,或先行词是that时 I have found that which I was looking for.