I am somewhat confused by the translation of litterās as the singular 'letter'. For example, in the Practice and Review of Wheelock's ch. 11, sentence #4 is given as
vosne easdem litteras ad eum mittere cras audebitis?
which is translated in Benissimus' key as
Will you dare to send the same letter to him tomorrow?
litterās looks to me like a feminine accusative plural (first declension noun). Is this simply one of those nouns (like ...