Why cannot I use magistram as well??
What would be a possible use for magistram???
Magistram is in the accusative case. You use the accusative mostly for direct objects or objects of prepositions.
Here's an example of magistram is a sentence: Cognosco magistram meam (I recognize my teacher).
why quis not quae?
You say quis when you are asking (i.e. you don't know who quis is). You say quae when you are describing someone.
Examples in English:
Quis: Who was the winner in the race?
Quae: The woman who won the race is my good friend.
Hope this helps.