"He hasn't entered his office for a month."
Translation:Il n'est pas entré dans son bureau depuis un mois.
with these 'done x since y' phrases, i thought you used the present tense initially "je travaille ici depuis un mois". why is this different? another case when the rule changes if there's a negation ? or is it because entering is not a continuous act ?
(update: the answer's here https://forum.duolingo.com/comment/36307760)
"Il n'entre pas...", in the present tense, is not as good as the passé composé "Il n'est pas entré..." because "entrer dans son bureau" is not a continuous action.
If the sentence were not negative, you would get "Il est entré X fois/tous les jours dans son bureau depuis un mois" (he has entered his office X times/every day for a month)
If you use "pendant", you don't know when the action started and you express a duration that is complete now. It is not the meaning of the sentence.
He hasn't entered his office for a month = Il n'est pas entré dans son bureau depuis un mois: This started one month ago and today, this is still true.
He did not enter his office for a month = Il n'est pas entré dans son bureau pendant un mois: This happened at some point in the past and we don't know anything about the current situation.