Translation:He translated the whole book in three months!
"in three months" of course sounds better to my English ears. But, pleasing my English ears is not the purpose of this course (often it seems quite the opposite ;-) .) Using "during three months" would seem to me to be more proper Esperanto according to what we've learned thus far. "en tri monatoj" immediately seemed incorrect.
In Bulgarian, I would use за, which is roughly por in Esperanto. Which would be better: „Li tradukis … dum 3 monatoj“ or „Li tradukis … por 3 monatoj“?
Obviously, por is completely wrong. And since Esperanto is presumably logical about prepositions (and because I'm not a native English speaker), en sounds equally absurd to me.
I vote for dum, which AFAIK specifies a duration of time, which is exactly what „in X months“ means in English. I believe this is the most logical preposition here.
If I may, I would like to say that IMHO both en and dum are acceptable prepositions to use in such an Esperanto sentence, but the first is the only one that translates accurately the English sentence.
The sentence reminds me of the project management concepts of effort and duration, or elapsed time. (It seems more plausible that it refers to the latter, because otherwise I think it would not talk about months but days or hours, which are more suitable units for a small project like this, but this is just my opinion.) Either way, according to my interpretation of PIV-o, no matter the units, both duration and effort can be better expressed in Esperanto with the preposition en than with dum, even though the latter would also be acceptable. Let's see why.
Here is the definition of en. (Non relevant parts have been concealed):
en. (PIV-o) I- Prep. montranta: […] 3 La tempodaŭron, interne de kiu okazas la ago: […]; 'dek horojn en la tago ŝi kudradis'; […] 'en la daŭro de unu monato'. 4 La tempolongon necesan por fari ion: 'legi libron en kvar tagoj; en momento la homo reviviĝis; vi forgesos nin en ses monatoj; en unu minuto li estus inter ili'.
You can see that the 3rd meaning of en relates more or less to the duration, while the 4th does to the effort. As Esperanto is not designed to fulfill the needs of project management, if it is one or the other will be subject to interpretation. But, in either case, the use of en preceding a time quantity should refer to an indeterminate period of time.
And now for the definition of dum:
dum. (PIV-o) I- Prep., montranta la tempon, en kies daŭro io okazas: 'fleksu arbon dum ĝia juneco; dum la manĝado venas apetito; dum la parolado li dormis; ŝia voĉo dum la parolado estis ne laŭta k tremanta; dum la vojo Klaro parolis al Marta'.
We can see that dum does not deal directly with time spans but with specific periods of time that contain the action. As I see it, this feature makes it necessary to specify the moment of time when the action takes place, even if it is called only by its duration. Clearly, we could use dum in a sentence like Li tradukis la tutan libron dum du monatoj de la somero! (“He translated the whole book in two months of the summer!”), or as well in Li tradukis la tutan libron dum du monatoj (“He translated the whole book during two months!”), where the time mentioned is not a duration but a concrete period in the calendar. (I am no English native speaker, but I would say that in this case the sentence cannot be translated properly into “He translated the whole book in two months!”) In both Esperanto sentences dum could be replaced by en (3rd meaning).
In conclusion, IMO:
in ≡ en
during ≡ en, dum
(I beg your pardon if all this makes not sense to you. I am still a komencanto.)