I'd say "on."
Playing in/on the playground?
I'd say "on."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
I'd say "in", or perhaps "at" but never "on".
How can I decide which one the correct is?
Different dialects.
Both are correct. I'm American. Ems is British.
You'll get different answers if you ask about "in" the street and "on" the street as well.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.