on the farm指的是在有明确坐标地点的农场中
in the farm指得是在平面上的农场，视觉是由远及近。
一、on the farm
1、He has twenty head of cattle on the farm.
他在农场养了20头牛。
2、They worked on the farm, tapping rubber trees.
他们在这农场做割胶的工作。
3、We have some chicken on the farm.
我们在农场养了一些鸡。
二、in the farm
1、I feed everyone in the farm.
我饲养农场里面的每一个(动物)。
2、The problem is that the new generation do not want to be or work in the farm anymore.
现在的问题是：中国的年轻人都不愿意在农场里干活儿了。
3、They spent a day and night in the farm of a very poor family.
他们在一户非常贫穷人家的农场里住了一天一夜。
