I wrote "I used to work on a farm" because that's how I've learned to say it and to me "work in a farm" sounds odd, even though I can see how they could mean the exact same thing. What do you guys think?
Any native speakers of English on here to explain the difference if there's one? I always think that big open places like farms are with "on" and for example houses and cities have "in -> he lives in New York" but "he lives on a big farm in Australia"
It's hard to come up with a general rule! I'd say it has to do with whether we perceive a place as enclosed or not, but those perceptions aren't very logical. For instance, it'd also be "in the desert", even though deserts are wide, open spaces too.
Probably, "in" is what we usually use, and "on" is used in specific phrases (like "on the farm", "on the street", "on the pitch"), or when you could replace "on" with "on top of" ("on the roof/on top of the roof"). If in doubt, go with "in" because it'll only sound a little bit weird if it does at all, and it's the kind of "mistake" that native speakers make too.
It's definitely "on the farm" though ;)