老师讲到that引导定语从句作介词宾语时，介词不可提到that前，介词后需用which或whom代替是什么意思？

并举例说明，谢谢

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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 .

比如：
This is the pen ( which /that) he wrote his composition with. 在这个句子中which/that可省。
解释：上面这句with介词在后，所以可以用that，或者which，并可省略。
This is the pen with which he wrote his composition.这个句子中 which 就不能省。
解乎虚释：上面这句with介词在前，所以不能用that，只能用which（这里指物，所以用which，如果指人，就用whom）

再比如：
I like the boy (whom/that) we played football with.
解释：上面这个介词在后，所以可以whom或者that，并可省略。
I like the boy with whom we played football.
解释：上面这个介词在前，所以不能用that，只能用whom（这里指人，所以用whom，如果指物，就用which）

另外补充知识：
在磨庆定语从句中，只能用that引导的情况如下　　
1、先行词是最高级或被最高级修饰。
This is the most interesting book that l have ever read.
2、先行词是序数词或被序数词修饰。岁游燃
The second foreign country that l expect to visit is Greek.
3、先行词是不定代词时，如"all、few、little、much、anything、nothing、something"
That’s the only thing that we can do at the moment.
4.、先行词既有人又有物时

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介词碧裂后接which或whom的定语从句相当于接whose、when、where等的定语从句,这类从句差蚂的结构虚慧埋是完整的。

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which是引导物的，whom是引导人的

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