Litteris is in the dative. Studere is one of those verbs that takes indirect objects (datives) when we English speakers would expect a direct object.
But dative hasn't (formally) been introduced here yet in the Tips, let alone the plural declension. It might confuse the beginner?? I think many people read the Tips first before going to the lesson.
Where exactly are these "tips" located? I don't have any in the browser version.
There was another sentence in this skill that does the same thing. One of the challenges is that the concept expressed in Latin here is expressed differently in English unless I've misunderstood something.
Is there a rationale behind it? What other verbs or kinds of verbs do this?
"A boy studies literature" was marked incorrect and corrected to "The boy studies literature"
Perhaps also accept "letters" (I know in the plural littera can be literature, but can also simply be a plural for letters, since it is a boy doing this, my thought jumped right to children learning the alphabet.)