"Buy candies, please."
Translation:Купи конфеты, пожалуйста.
I found a few interesting discussions regarding verb aspects in imperative constructs: http://forum.wordreference.com/threads/2252015/ http://russian.stackexchange.com/q/7329/31057 http://russian.stackexchange.com/q/1512/31057.
My understanding from these discussions is that:
The imperfective aspect (e.g. покупай here) emphasizes the action or the process itself (e.g. "do something"), whereas the perfective aspect (e.g. купи here) emphasizes the outcome (e.g. "get it done").
The imperfective aspect is used for general advice ("always do something"), the perfective is used for specific situations ("do this now").
It seems that occasionally one of the aspects may appear to be more demanding and impolite than the other aspect. I can't say which, because I read conflicting information in the above mentioned discussions. Perhaps it's inconsistent and varies with the actual verbs - I guess some experience and sensitivity is required to speak properly and politely ;)
Read about verb aspects (perfective vs. imperfective, e.g. http://www.russianlessons.net/grammar/verbs_aspect.php ). Покупай is imperfective whereas купи is perfective.
Which is what bothers me about this example when presented in the English to Russian form. Duolingo seems inconsistent with these. "Some" is seldom implied in English when talking about mass nouns. I would usually say, "Buy some candy or buy some flour" In these examples of English to Russian it becomes a guessing game because I would never just say "buy candies" in English. It seems sometimes when the English to Russian question in English is Imperative + noun the correct answer might be Imperative + genitive plural "конфет" or imperative + accusative plural конфеты as in this example. If the example is meant to be a mass noun, the word some should be used in English.