everything that 中that 引導的是定語從句，（先行詞是不定代詞everything, that 是關系代詞），例如：

She always sits at the front of the class , making sure that she can grasp everything that teachers say.

她總是坐在教室的最前面，確保能聽見老師說的所有內容（摘自《網易有道詞典》）（本句中，that teachers say 是定語從句，脩飾先行詞everything）

這是必須用that的情況

that常用作關系代詞，可指代某人，也可指代某物，指物時有的時候可用which替換that，指人時可用who替換。但在下列情況下，一般用that。

一、that指代某物事時

1.先行詞為all, few, little, much, the one, something, anything, everything, nothing等時。如：

（1）We’ll do all that we can to protect the endangered plants and animals.

我們將盡我們的最大努力來保護那些瀕危的動植物。

(2) There is much that I wan to tell you.

我有很多想要告訴你的話。

(3) Is there anything that I can do for you?

有什麼我可以幫你的嗎？

2. 先行詞被all, every, no, some, any, (a)few, (a)little, much等限定詞脩飾時。如：

(4) You can borrow any book that you want to read in our school library.

在我們圖書館，你可以借任何你想讀的書。

3. 先行詞被序數詞或形容詞最高級脩飾時。如：

（5）This is the most beautiful city that I’ve ever seen.

這是我見過的最漂亮的城市。