定语从句中的that在定语从句中充当主语时不可省略，做定语从句中的主语是必须用，但作为宾语时可以省略。

省略情况

1．用作代词，代替前面所指的事物，常见于表示比较关系的状语从句中，例如：

It is warmer in the south than ( that of )the south.

2．引导名词性从句作宾语，即宾语从句，例如：

She said ( that ) she would come.

I suggested ( that )we ( should ) go home.

3．作关系代词，在定语从句中作表语或宾语，例如：

Can I have the records ( that ) I lent you?

He is just the man ( that ) you talked about.

4．在 way （意为 manner ）后面，that 意思为 in which，例如：

Do it the way ( that ) I showed you.

5．在表示个人对事物反应、表示重要性或迫切性的adj后所接的从句中，例如：

I am glad ( that ) you are right.

It is obvious ( that ) they are happy together.

6．so that, so…that, such…that, 结构中，例如：

I came to see you so ( that ) you would know the truth.

She had so many children ( that ) she didn’t know what to do.

I was having such a nice time ( that ) I didn’t to leave.

7．用于表示时间的词后面，整个短语相当于when，例如：

I joined the army the year/month ( that ) my father died.

I thought her nice and honest the first time ( that ) I met her.

= I though her nice and honest when I met her.

8．如果定语从句中的谓语是 there is，作主语的关系代词 that 也常省略，例如：

This is the fastest train ( that ) there is to Beijing.