If I say: I live on a farm and I live in a farm, are both correct? And why "ON" a farm?
Prepositions in every language are very difficult to learn, and different in every language. It all depends how the culture views the world. Normally in English we say "I live on a farm".
Normally we use the preposition on with surfaces (on the floor/table/ground) as well as small islands (on Maui). A farm is a collection of many things (buildings, fields, animals, crops etc) and so in English (I would say) we view it much the same as an island or a large surface (the ground or land) and therefore we use on.
When we talk about more specific locations on a farm we can use in: in the farm-house, in the field, in a farm community.
Hope this mkes sense.