that 做何成分时可/不可省略

that 做何成分时可/不可省略
that做什么成分不可省略?
什么时可省略?
请分别举例,并指出that此时它在从句的作用.
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分成两种情况,一种that在名词性从句 ,另外一种在定语从句
定语从句：先行词在从句中充当宾语用that引导.可以省略.
可以省略：This is the biggest lab____we have ever built in our university.
A which
B what
C that
D where
选C built缺少宾语,用which/that 主句中有最高级只能用that
而且that可以省略,因为lab充当built的宾语
不可以省略：Is oxygen the only gas____helps fire burn?
A that
B /
C which
D what
选A 从句中help缺少主语用which/that 主句中有the only只能用that
that在从句中充当helps的主语.不可以省略that
名词性从句1、that在主语、表语和同位语从句中不可省略
2、that只在宾语从句中可以省略
可以省略：He said that he would go to beijing.
宾语从句的主句是不完整的.需要引导词引导的从句充当某一成分
that he would go to beijing.是said的宾语.that可以省略

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