[motoboi]

Duolingo uses spaced repetition. It's like Anki, except that Duolingo will give you some time after you successfully remembered a word or expression in an answer.

Don't know if that is your case, but if you get the "the cat is black" sentence and translate it wrongly, it assumes you are having difficulty with this expression and repeats until you get it right.

When you come to the point of commiting some words to you memory, it uses them to incrementally build grammatical concepts over there (like adjectives and verbs).

It's a pretty complex system going on behind that simple interface.