Learn Turkish: Çocuk salonunda oynuyor. - The child is playing in his living room.

It breaks down into three parts: salon (living room) + -u (his) + -da (in). Turkish requires the buffer letter -n- whenever you attach a case suffix (like the location marker -da) right after a third-person possessive suffix (like -u). So, it becomes salon-u-n-da.

Could salonunda also mean "in your living room"?

Yes! By coincidence, salon (living room) + -un (your) + -da (in) also makes salonunda without needing a buffer letter. You have to rely on context to know if the speaker means "in his/her living room" or "in your living room." Here, the English translation tells us it means "his."

Why isn't the word onun (his) used in this sentence?

In Turkish, the possessive suffix -u on the word salon is usually enough to show ownership. Adding onun to make it Çocuk onun salonunda oynuyor is grammatically correct, but it is mostly used if you want to strongly emphasize that it is his living room and not someone else's.

The verb for "to play" is oynamak. Why is it spelled oynuyor here instead of "oynayor"?