"I buy that important magazine each month."
Translation:Tiun gravan gazeton mi aĉetas monate.
'po' is absolutely necessary if a sentence has 2 numbers higher than 1. 'po' can be used only if there is at least 1 quantity expression in the sentence. The most usual translation for 'each month' est 'ĉiumonate' or 'ĉiun monaton' or 'en ĉiu monato'
It's good to see a translation using a different word order then SVO like english; esperanto's isn't free for nothing. This one is OSV, right?
Right. The course could more often show other word orders than the usual order, but then it would be too complicate to translate from Esperanto to English. Duo should then add all possible word orders in Esperanto.