If you read the other comments on this page, you'll learn that this is because it's not the accusative at all but rather the dative. [derp...]
It's plural because "littera" is literally "letter" as in "of the alphabet". The Latin word for "literature" is literally "letters".
YOU said on the above comment that we need the accusative, not the dative. Now you say it's NOT the accusative, but the dative! Which is it?!? Also, since this course is in English, so I'm assuming taken by either native English speakers or people fairly familiar with English, why don't you have the explanations in English. I studied English all the way through the college level, and we were never taught "dative" anything. If it's an OBJECT, or the object of the preposition, can you not explain it that way? I don't even understand in the slightest what declensions are, let alone you using different words for terms we already know!
My apologies. I wrote that a year ago and clearly I was confused that day. There are also comments here discussing the difference between "legere" (to read), which does take the accusative, and "studere" (to study, literally to dedicate oneself to), which takes the dative.
English does not really distinguish between the different kinds of object, but Latin does (and Spanish and Portuguese make more distinctions than English does). Learning Latin means learning declensions and declension patterns.
Here is a plain-English overview of what the cases are and how they work:
Latin cases, in English
Adjectives must agree in gender, number, and case with the nouns they modify, but they have their own declensions. Sometimes you get lucky and the adjective just happens to follow the same declension as the noun, but that is not a guarantee.
The accusative is the direct object of the verb.
I throw the ball. "The ball" is the direct object of the verb "throw". It receives the action of the verb.
The dative is the indirect object.
I look at the ball. "The ball" is the object of the preposition "at" (at least in English).
I give Livia the ball. "Livia" receives the direct object, so "Livia" is the indirect object.
The "da" in "dative" is the same "da" in the Spanish word "dar/to give".
Damelo! Give me that! (Or "Give that to me.")