60天口语社群有同学提到了一个定语从句：One trend we're noticing with nearly all age groups is the growing popularity of holidays in whichclients do some kind of specialised activity.
句子中in which应该如何理解？
这其实是定语从句中一个很常见的知识点：介词前置。上面的句子可以拆为两个分句：
(1) One trend we're noticing with nearly all age groups is the growing popularity of holidays.
(2) Clients do some kind of specialised activity inholidays.
这两个句子合并为一个，就变成：
One trend we're noticing…the growing popularity of holidays which clients do some kind of specialised activityin.
以前的语法学家认为，介词不能放在句子最后面，因此人们通常将介词提前，句子就变成了：One trend we're noticing…holidays in which clients… 其中which指代holidays
又比如有这么一个外刊句子：
The aim of letting the world warm no more than 1.5°C, agreed on in Paris six years ago, is oneto whichmany countries, notably the small-island statesfor whichsea-level rise is an existential threat, are deeply committed.
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（6年前，各国在巴黎定下将全球气温上升幅度控制在1.5℃以内的目标。很多国家都在尽力完成这一目标，因海平面上升而面临生存威胁的小岛国尤为如此。）
句子主干是：The aim is one (aim) to which many countries are deeply committed. 这里也出现了介词to提前的现象，它来自搭配be committed to something，而which指代one
the small-island states for which sea-level rise is an existential threat也是同样的道理，正常语序是sea-level rise is an existential threat for the small-island states，which指代the small-island states
那么，介词能不能放在句子最后面，而不提前呢？实际上这种写法也是可以的，比如我们可以说：There are several small ponds in which a variety of fish live.
也可以说：There are several small ponds which a variety of fish live in.
但《剑桥词典》指出，正式文本里面通常将介词放在which前面。英语新闻和学术论文里面一般采用这种写法。在口语和非正式写作里面可能会出现介词放在句尾的用法。
我们在写考场作文的时候也要参考正式写法。
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