on/at/in the farm
 【问】 要想表达“在农场里”的意思，只能说on the farm吗?
 【答】 既可以遇到on the farm的用法，也可以遇到at the farm的用法。例如：
 1. During the university vacation he worked on a farm. (Wood)
 大学假期期间，他在农场里工作了。
 2. My father raises honeydews on his farm near Jenkings town. (Caldwell)
 我父亲在詹金斯镇附近的农场里种甜味烟草。
 3. About ten years after they left you with me，a fire broke out on their farm. (Queen)
 在他们离开你和我之后，大约过了10年，他们的农场发生了火灾。
 4. Then he remembered that he was at the farm and that he was going out to take pictureswith his father. (English This Way)
 那时他还记得他在农场，他要出去跟他爸爸照相。
 根据Wood的说法，当在农场直接从事农业劳动时，必须用on：
 1. We are going to grow rice on the farm. (Wood)
 我们打算在农场种稻子。
 2. First he had worked on a farm. (Wright)
 他最初在农场工作。
 如果所指的不是直接从事农业劳动，而是在农场里做户内工作，则常常使用at：
 At the farm，the plumbers are busy at work.
 管子工在农场里忙着干活。
 当指某个特定的农场时，也可以使用at：
 That is the farm at which I used to work. (Wood)
 这就是我从前工作过的农场。
 当表示在农场居住、停留时，除on，at之外还可以使用in：
 1. Mary likes to stay with her uncle on his farm. (JSD)
 玛丽喜欢同叔叔住在他的农场里。
 2. She had been born ten miles to the south，on a farm she and her mother had loved.(Updike)
 她出生在南面离这10英里的一个农场里。她和妈妈都很喜欢这个农场。
 3. He was born in 1809，in a small farm in Kentucky...(Eckersley)
 他于1809年生于肯塔基州的一个小农场……