介词在疑问代词前的情况

介词通常位于名词之前构成介词短语，但在一些句式中，如果介词与作其宾语的疑问代/副词或关系代词关系密切时，介词可以移到前位。

1. 在疑问句中
Since when have you been working for your present employer? 你从什么时候开始为现在的雇主工作？
In what forms does energy exist? 能量以什么形式存在？
At which station should I get off? 我应该在哪个站下车？
With whom do you wish to speak? 你想和谁说话？

2. 在定语从句中
Near where I live there's a pond in which you can find many fishes. 我住的地方附近有一个池塘，里边有很多鱼。（注：这里 in which = where)
I remember the day on which I met him for the first time. 我还记得我第一次见到他的那一天。
That's the ship on which we travel (=That's the ship we travelled on. ) 那是我们旅行坐的船。

3. 在非限制性定语从句中
The fancy-dress party, at which the men all turned up as gangsters, was held in Manhattan. 化装舞会是在曼哈顿举行，这场舞会中男人都扮成黑帮成员。（注：这里 at which = where)
This is Mr. John, for whose son I brought a book yesterday. 这是约翰先生，昨天我给他的儿子带了一本书。

4. 在宾语从句中
Let us know on which train you will arrive. (= Let us know which train you will arrive on). 让我们知道你将乘哪趟火车。