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.
