I am at beginner level of Russian and I have come across with a phrase type that I have not understood it yet. The phrase "I work as (a) teacher" is translated into Russian as "Я работаю учителем". I dont understand why is the name of job's agent used in instrumental case? As a person who uses English (and natively Turkish) I tend to say "Я работаю как учитель". When I had described what I do in Russian as "Я работаю в университете как научний помощник" it was corrected in this way "Я работаю в университете научним помощником". My Ukrainian friend has said that it is related to the question "Кем ты работаеш?" because кем is used in instrumental case. But if one tries to translate it literally into English, it is like "I work with/by teacher" that it is nonsense for me. Can anyone explain what I miss?