我想问一下关于定语从句的介词前移的问题
fair is an event at which companies present and
sell their products .这里的 at 前移到which的前面，有时候不是很明白，希望学长举例说明一下。并详细说明一下，thank you so much。

fair is an event at which companies present and
sell their products
商品展览会是一个商家展示和促销其产品的场所。
at which companies present and
sell their products是后置定语从句，which指代event，at which 作从句的前置地点状语。为什么要前置？这是定语从句语法所要求的，是为了将修饰语和被修饰语靠近而不至于产生误解。

请看以下词典释义中的黑框部分：

【event】[countable] a performance, sports competition, party etc 【at which】 people gather together to watch or take part in something:
The conference was an important social event (=an event 【at which】 people can meet each other

下面再举数例：
关系代词WHICH/WHOM在定语从句中做介词的宾语时,从句常用"介词+WHICH/WHOM"引出。例:
1）Great changes are taking place in the city in which they live.
2）This is the teather from whom we've learnt a lot.
3）The comrade with whom I came knows French.（whom在从句中作介词with的宾语）

以下规则要记住：

关系词的概念是:既要起连接作用,又要在从句中做成分。

关系代词和关系副词的用法：1）当先行词为人时用who 作主语，whom作宾语；2）当先行词为物或整个句子时用which ，可作主语或宾语；3）先行词为人、物时用that ，可作主语或宾语；4）whose用作定语，可指人或物；5）关系副词when（指时间，在定语从句中作时间状语，where（指地点，在定语从句中作地点状语），why（指原因，在定语从句中作原因状语）。

介词在关系代词前，只能用which和whom，且不能省略；介词在句尾，关系代词可有which, that, whom, 口语中也可用who，且可省略。