这主要是因为英语的动词词组分为两种：动词短语和短语动词。动词短语是可以把介词提前和动词分开的，词组意思不变，如learn from:lily is the girl from whom we should learn.
还有一类短语动词，介词是不能和动词分开的，否则词组意思改变，这时候介词就不能提前。如this is the man who we are looking for.
注意：介词提前时，定语从句的引导词只能用 which, whom, whose,而不能用that或who。
因为关系代词在从句中做介词宾语，所以前面要加介词，以“介词+关系代词”的形式来引导从句，如：
This is the school in which I studied two years ago.
但有些是固定搭配，如look for,look at,take care of等，如：
The book which I am looking for.
their ship was decommissioned, and
Paul bet that
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