"The boy studies literature."
Translation:Puer litteris studet.
Unlike what PERCE_NEIGE said, it's not an exception. It's simply a matter of the semantics of the verb studere. It doesn't actually literally mean "to study". It literally means "to dedicate oneself to". The fact that it's "dedicate [oneself] to" (as opposed to just "dedicate") means that it must call for an indirect (ie, dative) object as opposed to a direct (accusative) object. The accusative element, "oneself", is baked right into the verb, so no direct object is necessary, and only an indirect object is required. Hence, the dative case.
'Litteras' would be used in the accusative case. In this sentence, the verb 'studeo' makes the verb's object (litterae) turn to the dative case, which implies in the -is ending. You could make use of a sentence with -as ending in, for example, cōtīdiē tuās litterās exspecto (I expect your letters daily).