Simple past vs past progressive in as/while/when-clauses

Question

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

1. 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?

2. 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).

Answer 1

1. As and while in their temporal senses^† constrain imperfective readings because that’s part of their semantics: both mean approximately “during” a timespan defined in the remainder of the clauses they head. The matrix clause which the As or While clause modifies is seen as taking place within the continuing action which the As or While clause describes—which is thus by definition imperfective, since neither its beginning nor its end lies within the timeframe of the matrix clause.

2. You may encounter As or While temporal clauses with telic verbs in the simple past.

   While I left the beach, I saw a UFO.

   In this case, yes, while constrains an imperfective reading of left. But it’s a use that I think most native speakers would find more than a little ‘off’.

   • The simple past is by default perfective.
   • Perfectivity is more strongly marked with verbs which can be ‘perfected’, viz., telic verbs.
   • It is even more strongly marked with LEAVE, whose lexical aspect is not merely telic but very close to cessative, explicitly marking the end of a state.
   • And at the same time as these factors are resisting the necessary imperfective reading in the While clause, the verb in the matrix clause, saw, is a stative which by contrast inherently prefers an imperfective reading.

   So I think this sentence unlikely to occur except in speech where the speaker is groping for words. Most people would say

   While I was leaving the beach, I saw a UFO.

   But as I explained in the previous answer, a While clause with an activity verb takes a simple past more readily, particularly if the matrix clause employs either an activity verb or a telic verb which is syntactically constrained to a repetitive reading:

   While I ran on the beach, I saw several UFOs.

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^†I did not make clear in my previous answer that I was addressing only temporal clauses. The situation is of course quite different when these conjunctions are used in non-temporal senses, roughly equivalent to because and although respectively.