I was given a link to this Q and A board by a member of English Language & Usage and this issue has been rattling around in my head for almost a day now.
The question Simple past vs past progressive in as/while-clauses by Sherlock has this answer by StoneyB, and the answer made me think of some new questions
In the answer, StoneyB writes In this example it makes no difference, because both as and while constrain an imperfective reading of the verb. Why do both as and while constrain an imperfective reading of the verb?
Can the "when"s in the answer's example sentences When I left the beach, I saw a UFO. and When I was leaving the beach, I saw a UFO. be replaced by "while"s and if yes does this make them both imperfective?
To me it seems that in the first paragraph of the answer the aspect depends on the as/while clause whereas in the second one with the when clause on the verb itself (telic or atelic).