1、为什么并列句前后都有that

但是which在句中充当成分，一般不可以省略。

(相关资料图)

例如：I said (that)he was right

He asked me which was his

that引导宾语从句无意义，不充当句子成分常省略。但下列情况除外：

①．介词宾语从句的that不能省略。

He is a good studentexcept that he is a little careless

②．and连接的两个从句，两个从句的that不能省略。

He told me that he hadtwo sons and that they both had gone to college.

③．在动词+it+宾语补足语+宾语从句结构中，that不省略。

I heard it said that hehad gone abroad

We found it impossiblethat he could finish it in such a short time

that引导的宾语从句中,that在句子中不做任何成分,在口语中还常常省去，但如果从句时并列句时,第二个分句前的that就不能省略,

而which呢,引导的宾语从句相当于特殊疑问句,应注意使用陈述句语序就行

而两者区分的关键呢就是看在句中是否做句子成分

2、that在句子中为什么可以省略

that是强调句的标志，是不能省的

It is that是强调句的固定用法:

一、如果强调句型指现在或未来情况用It is，指过去情况用It was。

二、被强调部分往往是句子的主语、宾语、时间状语和地点状语，但有时也可强调比较复杂的内容，如时间状语从句、条件状语从句、not until…、not only…but also…、…as well as…等结构。

3、什么情况下句子要加that

1，当先行词为 all, anything, everything, nothing, something, few, little, much, any 等不定代词时，如：

Do you have anything that must be claimed?

你有东西需要申报吗？

I"ll do all that is the party needs me to do well.

党需要我干什么我就干好什么。

2，当先行词被 all, any, little, no, few 等词修饰时，如：

There are few rules that have no exception.

很少规则没有例外的。

All the poisons that are absorbed by the body are brought to the liver.

身体吸收的有毒物质都被送到肝脏。

3，当先行词为形容词最高级或被最高级修饰时，如：

This is the highest score that I can get.

这是我所能取得的最高分数。

This is the most interesting book that I’ve ever read. 这是我读过的最有趣的书。

4，当先行词被序数词修饰时，如：

The first thing that needs to be done isto wash your hands when you go home.

当你回家时第一件要做的事是洗手。

The second approach that we’ve taken doesn’t work. 我们采取的第二种方法不起作用。

5，当先行词为数词或被数词修饰时，如：

The two articles that I recommend to you are written by Mark Twain.

我推荐给你的两本书的作者是马克·吐温。

There are three that we found in that room.

我们在房间里发现了三个人。

6，当先行词被 the very, the only, the same, the last 等修饰时，如:

He is the very man that we need .

他正是我们需要的人。

That"s really the only thing that hurts.

只有这件事真的令我伤心。

7，当句子的主语为which 时，如：

Which is the product that we desire?

哪一个是我们所期望的产品？

8，当先行词同时包含人和物时，如：

The old man and his daughter that we saw yesterday did not come here again.

昨天我们见到的那位老人和他的女儿没有再来这里。