Do the three sentences below have any differences in meaning? Do I use simple past or present perfect or simple present tense when it is a general scenario?
Every time I go back to my dorm from a class, I look up words the professor mentioned and said during the class.
Every time I go back to my dorm from a class, I look up words the professor has mentioned and said during the class.
Every time I go back to my dorm from a class, I look up words the professor mentions and says during the class.
First I'd like to say that either "mentioned" or "said" is sufficient. They more or less mean the same, so you don't have to use both.
I would prefer sentence nr.2 to express that what the professor has said is important to me and that is why I look it up later. It is fresh in my mind and is important even after the class has finished. That is why present perfect is used.
Having said that, many people would use sentence nr.1, especially in AE. In this sentence you also see "during the class" as finished time, that is why you use simple past.
Sentence nr.3 sounds like you look up the words at the same time the professor mentions them, which is not true. You look them up afterwards. So that sentence is not correct.
