a farm前跟介词为什么用on?

a farm前跟介词为什么用on? farm 前用什么介词? my uncle works on a farm now. 什么用介词on 而不用at on the farm =at farm前的介词怎么用· 求介词+某地点的固定用法.还有在农场里为什么要用on a farm?而不用In. Mr.Smith has many pigs on his farm.为什么要用on这个介词? uncle works on a farm.这里的介词用on对吗? a farm 前面用什么介词~重赏! on the farm 为什么用on 为什么Christmas 前用介词at,而teacher's day 前用介词on lake前的介词用in 还是 on 为什么? He was raised by his grandmother on a farm.为什么是on a farm?