"I live on the first floor."
Translation:أَسْكُن في ٱلْطّابِق اَلْأَوَّل.
There is a slight difference in the meaning when you omit "في" here. The one in the exercise means "I live in the first floor. (And others might be living there as well)", you translation would meam "i live in the first floor. (and i have it claimed all to myself and no one else can live in the first floor)"