litteris = Dat. Pl., Abl. Pl. Should we here do not use the accusative. (therefor: litteras)
studeo, studere usually takes the dative instead of the accusative for the 'direct object'. There are a few other verbs that are like that but I don't remember it being a huge amount.
It is easier to understand why it takes the dative, when you find out that its basic meaning is "to devote oneself zealously to..."
Isn't both "who study" and "who studies" correct?
No, because u dont say he study, just I study
Like i act, he acts