that在名词性从句中不作任何成分,无意义,是个连词,不能省略;定语从句中的that是关系代词,在定语从句中作主语、宾语;作宾语时,可以省略;若介词提前时,不能用that,可以用which或whom。
That he is ill is true.
This is the city (that) you mentioned.
The car that is to be repaired is mine.
I still remember the day on which I joined the army.
that的用法:
一、that 作限定词或代词
that可以作限定词或代词连接名词或名词短语,也可单独使用,用以指代人或者物。此外,that不仅可以指代单独的人或物,还可以指代一句话。
二、that作副词表示程度,相当于so
例:It isn't all that cold.
天没有那么冷。