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.
