We do the opposite. If something is known we just say it.

“The girl(1) is playing(2) violin(3) downstairs(4)”

دختر(۱) در طبقه‌ی پایین(۴) ویالن(۳) مینوازد(۲)

To refer to an unknown thing we add a prefix ی to the thing

A girl -> دختری
The girl -> دختر