关系代词whom,which,that在限制性定语从句中作动词宾语或介词宾语,而该介词由位于从句末尾时才省略,

在定语从句中,关系代词that ,which,whom 在定语从句中作宾语宾语时可以省略,例如
He found the book ( which/that ) he lost yesterday。
This is the boy (whom ) I talked of the other day。
如果定语从句中出现把从句中的介词提前到关系代词which和whom 之前,这时候关系代词which和whom 作介词宾语时,关系代词不能省,介词提前关系词不能用that 。
This is the pen ( which /that) he wrote his composition with。在这个句子中which/that可省。
This is the pen with which he wrote his composition。这个句子中 which 就不能省。
I like the boy (whom ) we played football with。
I like the boy with whom we played football 。
再问： 能举个AS的例子吗
再答： as 引导的定语从句，其先行词为整个句子的内容，在定语从句中充当主语或宾语， 其实就相当于一个非限定性的定语从句，这时关系词是不能省略的。as 引导的定语从句位置比较灵活，可放句首，句中或句末。 eg, As we all know /As is known to all , the matter has three states。 The matter has three states, as we all know /as is known to all 。 The matter , as we all know /as is known to all , has three states。