Difference between the tenses

Question

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"

Accepted Answer

Hola Ashlyn
This type of sentence normally uses "era" as in "used to be" (another use of the imperfect) because when we talk about friends in the past, this is normally what we convey, however you can also use "fue" here if you see this as something that happened in the past and had an end, even if that end is not explicit in the sentence, but it is how the speaker is seeing this action. It happened and it ended. With era, on the other hand, even though that action is also something that happened in the past, you are expressing that action as something where there is no relevance or emphasis on an end. It's more descriptive. 
Saludos
Inma

Answer

Hola Johathan
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.
Saludos, Inma

Answer

Thank you Inma..I think I'm starting to get it! 
Saludos, Margaret

Answer

...this is an excellent example of what a native English speaker would have to say to convey the intention of the spanish imperfect.

Answer

Jonathan's rule is indeed sometimes helpful - i.e., if you can substitute "was/were [running]" into the sentence such that it still makes reasonable sense, then use the imperfecto. Another acceptable guideline might be to ask: "Does it fit the expression 'used to …'?" - e.g. one of the examples in this lesson could perhaps be translated: "Antonio used to eat a sandwich at midday", so use the imperfecto. However, a word of caution - these rules will not usually work with the English verb "to be" [see below >>] (and often, not with an English sentence containing "had")…. Another [for me, useful] 'rule' is that descriptions should always be rendered in the 'imperfecto', e.g. if we want to translate "the house was very big", then we should not try and invoke the "was being" rule to obtain "la casa era muy grande" >> because "the house was being very big" sounds absolutely terrible, and "the house used to be very big" changes the meaning to an incorrect one.

Answer

there was a question.Él ________ mi amigo.He was my friend(HINT: Conjugate "ser" in Pretérito indefinido)should this be imperfect instead of simple past tense since it did not specify a start and end date, and also sound descriptive.