The difference between entro / in / tra-fra is a little bit tricky. The table above is a summary.
1)
Entro corresponds to the English preposition until. In this case it refers to an instant of time. (Deve farlo entro domani / He must do it until tomorrow). It can be used as well as the english preposition in (Deve farlo entro una settimana / He must do it in one week). But in both cases the meaning is that it must done before, before tomorrow and in less than week.
2)
In case of a period of time entro can be substituted by in.
Lo faccio in una settimana.
Lo faccio entro una settimana.
I do it in one week. / I do it before a week is over.
In relationship to an instant of time entro can only be substituted by fino.
Lo faccio entro lunedì.
Lo faccio fino a lunedì.
I do it until monday.
3)
The problem is the English preposition in because it is ambiguous.
I do it in one week.
=> I do it before a week is over.
=> I do it after a week is over.
The Italian in is not ambiguous.
Lo faccio in una settimana.
I do it before one week is over.
The Italian preposition in can therefore only be translated with the English preposition in if the event / actions happens before the period of time is over.
4)
If the event is going to happen after the period of time is over the preposition tra / fra has to be used.
Lo faccio tra una settimana.
Lo faccio fra una settimana.
I do it after one week is over.
Select the right options, sometimes there are several correct options.