引导宾语从句的that 通常可以省略

其他连词不可以省略。

1.当that在从句中作补语时可以省略。

He is not the man (that) he used to be.

She is all (that) a teacher should be.

2.在there be 结构中出现定语从句，或在定语从句中出现there be 时，用作主语的关系代词也可省略。

There is a man downstairs (who) wants to see you.

This is the best dictionary (that) there is in the library.

3.状语的省略

（1）当先行词是reason，而且定语从句中作原因状语时，关系代词可用why,that,也可以省略。

The reason (why/that) he failed was his laziness.

That is the reason (why) I did it.

（2）当先行词是way,且在定语从句中作方式状语时，关系代词可用in which,that,也可以省略。

The way (in which/that) these comrades look at problems is wrong.

That was the way (in which/that) she worked the problem out.

注意：当关系代词在定语从句中作主语时，不省略。

例如：I don't know the way that/ which leads to the top of the mountains.

(3)当先行词是time时，关系代词可用when,that 或省略。

The second time (that) I saw him was in 1980.

I don't know the exact time (when/that) the sports meet will take place.

that 不能省略的情况——

1、and连接两个宾语从句，that宾语从句放在and的后面时，that不能省略。

2、that引导的宾语从句作介词宾语时，that不能省略。

3、that从句位于句首时，that不可省略。

4、主句谓语动词与that从句之间有插入语，that不可省略。