"They are washing the potatoes."
Translation:Sie waschen die Kartoffeln.
i experimented with the order and wrote: "die Kartoffeln waschen sie" and it was marked wrong. Is it really wrong -- and if so why? -- or just an answer that duolingo wasn't expecting?
It's a possible translation, if you want to emphasise either the subject, the object, or the verb -- in English, you might use a cleft for this: "It's the potatoes that they are washing" or "It's they who are washing the potatoes" or "What they do to the potatoes is washing them".
This course generally teaches words used in most of Germany and pays little attention to regional vocabulary or words used nearly only in Austria or Switzerland.
So use Kartoffel here rather than Erdapfel, Grundbirne, Krumbir, ....
Because den Kartoffeln is dative case, but waschen is an ordinary transitive verb that takes a direct object in the accusative case -- hence, die Kartoffeln (accusative plural).