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.
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