what's the different betwen at the farm and on the farm?
'at' as in 'He's not here. You'll find him at the farm' gives the general location. the whereabouts, of the person.
'on' in 'on the farm' indicates the area within the boundaries (the fences) of the farm, and on the surface area of the farm.
A person is at the farm, and working on the farm. He has cows on his farm.
A person is at the seaside, walking on the beach.
at sea, on a ship
at the theatre, on the theatre stage