"Corinna has a teacher."
Translation:Corinna magistrum habet.
The case, how the noun functions in the sentence.
magister is nominative singular (the subject, the one doing the action).
The teacher teaches Corinna. -> Magister Corinnam docet.
magistrum is accusative singular (the direct object, the one being directly affected by the action).
Corinna teaches the teacher. -> Corinna magistrum docet.