Would another way of explaining the difference be to say that in English you could get the meaning of the imperfect by saying "The teacher [was] opened[ing] the door" and the indefinido as "The teacher [had] opened the door"
That is a very good point. You could also use that theory to explain the difference but there are more tenses involved in that, for example, for the "was opening" sentence we'd probably use "estaba abriendo" as well as "abría", and "abrió"(opened) is not the same as "había abierto"(had opened). What we want to emphasise in this lesson is that in English the imperfect (abría) and the indefinido (abrió) are both translated as "opened", whereas in Spanish there is the nuance of an action with a beginning and an end or not.
Thank you Inma..I think I'm starting to get it!
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