If I am solving a case study for a class and I had to mention the questions asked by the authors above or below my answers in a Powerpoint presentation. I made it clear that the questions are asked by the authors of the case, however they were not put into direct quotation marks (questions were, obviously, copied verbatim). Is this a form of plagiarism? (I have added quotes later on and resent to the instructor just in case)
I don't see a strong case for plagarism here. According to Google:
[Plagarism is] the practice of taking someone else's work or ideas and passing them off as one's own.
It doesn't sound like you've tried to do this if: despite not being in quotation marks, the quotes were clearly indicated as the work of others. Furthermore you've corrected your mistake which shows good willing. I'd rest easy and wouldn't lose sleep over the matter.