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.
So you mean 'quae' is a relative pronoun, and 'quis' is a interrogative pronoun?
quae can also be the interrogative adjective. Quae femina studet? -> 'Which woman is studying?'
Not to mention the plural usages.
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).
Is magistra genetive
Magistra here is nominative.