Dear Rabbi Kornfeld,
Chullin 52A seems to indicate it is impolite to ask questions of your teacher. Why would this be so? (I would like to feel free to ask any reasonable question of my teachers.)
Shavua tov v'hag Pesach sameach,
Yehuda
There is generally nothing wrong with asking questions of one's teacher. However, there are some exceptions. If the teacher is not expected to have an answer to the question, for example, if one is learning a Masechta in Taharos which most people are not familiar with, he should not ask his Rebbi an in-depth question on the material unless his Rebbi is familiar with the material. The reason for this is that it will lead to the Rebbi's embarrassment when he must admit he does not know the answer. There are other times one should not ask his Rebbi questions, such as not peppering him with questions the second he walks in, and not bothering him with a question when he is clearly not in a mood to answer such questions.
All the best,
Yaakov Montrose
With regard to not asking when the teacher is not expected to have an answer to the question, see Chulin 6a for a source for the prohibition.
In addition, someone who is not "Bar Pachsi" ("Ben Gedolim"), perhaps should not ask a question of a Rabbi who is much older than him.
As an aside, in the case of Rav in Chulin 52a, since he was also a Tana, he would speak very tersely, more so than a regular Amora, as per Chulin 63b which interestingly is brought in the name of Rav. Therefore his Talmidim were careful not to ask extra questions.
Kol Tuv,
Yitzchok Zirkind