Just to supplement Jim's answer:
The preposition en, in a temporal context, means "within":
On a fini nos devoirs en une heure. -- We finished our homework within one hour.
Dans, however, denotes a time in the future, so you need future tense for this to work:
On finira nos devoirs dans une heure. -- We'll finish our homework in one hour. That doesn't mean that it took you one hour to do the work, just that you'll be done in one hour.
I am unable to locate the question in the first line of your query.
If we write "On a fini nos devoirs EN une heure" this means that the DURATION of the action was one hour.
If we write "On finira nos devoirs DANS une heure" this means that it is predicted that the action WILL TAKE one hour.
So in this context "en" is synonymous with "pendant" --> elapse time.
and "dans" is synonymous with "delay until a future action"
Hope this helps.
I have a follow-on question. For this same quiz question: On a fini nos devoirs ________ une heure. (We finished our homework in one hour). My answer was "pendant". It was marked incorrect and the correct answer was "en". My thought process was that the homework was completed during a timeframe of one hour.
After reading Jim's response, it sounds similar to what I was thinking. So shouldn't "pendant" also be a correct answer here?
Taking your translation of "On a fini nos devoirs ____ une hour" as "We finished our homework in one hour" then in this context, I would say that "durant" would be more appropriate in the sense of "au cours de". "We finished our homework during the period of one hour" so then we are back to using "en".
You make an interesting point but, on balance, the context focus is on "the duration of the action."
However, it would be interesting to receive a staff expert's input on this aspect.
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