定語從句中的that在定語從句中充當主語時不可省略，做定語從句中的主語是必須用，但作為賓語時可以省略。

省略情況

1．用作代詞，代替前面所指的事物，常見於表示比較關係的狀語從句中，例如：

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

2．引導名詞性從句作賓語，即賓語從句，例如：

She said ( that ) she would come.

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

3．作關係代詞，在定語從句中作表語或賓語，例如：

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

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

4．在 way （意為 manner ）後面，that 意思為 in which，例如：

Do it the way ( that ) I showed you.

5．在表示個人對事物反應、表示重要性或迫切性的adj後所接的從句中，例如：

I am glad ( that ) you are right.

It is obvious ( that ) they are happy together.

6．so that, so…that, such…that, 結構中，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．用於表示時間的詞後面，整個短語相當於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．如果定語從句中的謂語是 there is，作主語的關係代詞 that 也常省略，例如：

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