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


Yes. Almost. We have a special prefix for unknowns to make it simpler. Read my response above