Translation:These are bought at the shopping mall.
"One buys these…" "You buy these…" "They buy these…" are all acceptable English translations. Get it together, Duolingo.
why is the third person plural "comprano"-they buy, translated into the past tense-are bought? Why not "questi si hanno comprato"?
See Viaggiatore's comment above. 'si comprano' = 'are bought'
'Are bought' is not past tense, it's passive tense. Past tense would be 'were bought'.
I Am Not a Grammar Expert. But yes, the construction in English is certainly Passive. Passive (and Active) are Voices, not Tenses. The Italian here looks like a Reflexive verb though - comprarsi? Perhaps someone who is an expert can explain more!
Google translate tests: the English of what DL want (these are bought at...) gives "questi sono acquistati presso il centro commerciale". Totally different!! Pop in the italian DL uses here (questi si comprano al...) and, sure enough, we get back to DL's English. I'm guessing an Italian speaker would be able to advise..? Please!!
Since 'questi' is plural, I'm curious as to why it's not 'Questi ci comprano' rather than 'Questi si comprano'.
The difference between "ci" and "si" has nothing to do with the difference between singular and plural. "SI" is the right pronoun here because it's the third person reflexive pronoun. Si lava = He washes himself; si lavano = they wash themselves; but ci laviamo = we wash ourselves. The reflexive form is also used to form the passive: Si lava = he is washed; si lavano = they are washed. The passive is what you have here.
"Trade center" just sounds goofy, and has no immediate meaning to a native speaker.
I know what a "shopping mall" is. I know what a "commercial center" is. I do not know what a "trade center" is. I could probably figure it out by context, but it is not something I would ever say.
I am not a native speaker but where I live (Armenia) trade center is often used as a synonym for commercial center. If it sounds unnatural for native speakers, then okay. I had no idea.
We have a few "Trade Centres" in South Africa - they focus more on factory shops and light industries though and are not the same as shopping centres. The word "mall" is an interesting one - it was originally a kind of pedestrian street, hence, The Mall and Pall Mall in London!