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

As/while I was walking along the beach, I saw a UFO.

In that sentence, what difference does it make if the past progressive verb is replaced with a simple past?

As/while I walked along the beach, I saw a UFO.

Answer

In this example it makes no difference, because both as and while constrain an imperfective reading of the verb. That is, even though past-form walked is ordinarily understood as perfective (not 'perfect'), a completed action, use with as or while forces it to be understood as an activity in progress.

With when, however, it might make a difference, depending on whether the verb in the when clause is telic or atelic—has or does not have a final change of state ‘built in’ to its meaning. The verb in your when clause is atelic, so again it makes no difference in your example. But if the verb is telic—leave, for instance, which involves a change of location—it does make a difference.

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  With a telic verb, the past form is understood as perfective in a when clause; in that case, the action in the main verb will be understood to to occur after the completed action in the when clause:

  
  

  
  

  When I left the beach, I saw a UFO.

  
  

  
  


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  But if the verb in the when clause is atelic, or if it is a telic verb cast in the progressive form (which ‘stativizes’ it), it is understood as imperfective, an action in progress, and the action in the main clause will be understood to occur during the non-completed action in the when clause:

  
  

  
  

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