"We have a teacher."
The 'mouse-over'(?) presents the following as translations for "teacher": magistrum / magistra / magistram Using the accusative would just be either 'magister -> magistrum' or 'magistra -> magistram', right? So... Where, oh where, does that "magistra" in the middle come from?
Magistra is the nominative case and would be used should the 'teacher' be doing the action of having: magistra discipulos habet ('The teacher has students').
Since we are doing the action of having is being done to the teacher we need to use another case, the accusative case, which is magistram. If this was also not accepted, report it.