The accusative case in Latin shows which noun is direct object of the
sentence. The direct object receives the action of the verb. In
English, the direct object follows the verb. In Latin, the accusative case
endings show which noun in the sentence is the direct object no matter where it
is. In the following chart, make sure that you notice the difference in
endings between the nominative and accusative cases.
Predicate Nominatives vs. Direct Objects
All sentences have two parts - the subject and the predicate.
The subject (nominative) is who or what the sentence is about.
The predicate tells what the subject does or is. It includes the verb and its objects.
When you have an action verb, a noun that follows and answers who or what is called the direct object (accusative). e.g.
Elisabeth killed a spider. In this sentence, Elisabeth performs the action and the unfortunate spider
received it directly. In Latin, the noun spider would be in the accusative case.
Rule I. When you have an action verb, you have a direct object which is in the accusative.
However, you could have a sentence with a linking verb (am, is, are, was, were, etc.). When this happens, a noun that comes after the verb is not being acted upon. This noun renames the subject. e.g.
Sarah is a student. In this sentence, Sarah is the subject. But
is a student does not show action. It tells us who Sarah is. It renames her. We could even say
The student is Sarah and have it mean the same thing. A linking verb is the same as =. In the same way that 2+2=4 and 4=2+2,
Sarah = student and student = Sarah. The equations must be equal. So, in a sentence, if the subject is nominative, a noun
that comes after a linking verb, which is equal to the subject, must also be in the nominative to balance the equation.
There is also another special case that may arise when you have an adjective by itself after a linking verb. e.g. Shaquille is tall. In this sentence, Shaquille is the subject or nominative. Tall modifies Shaquille. Since adjectives and nouns must agree in case, number, and gender, tall (which modifies Shaquille, the subject) has to also be nominative in order to balance our equation.
Rule II. After a linking verb, any word that modifies or renames the subject must be in the nominative case.
How does this work in Latin?
It is the exact same in Latin. e.g., Puella aquam portat. Puella is the subject.
Aquam portat is the predicate. The noun aquam receives the action of
puella portat and has to be in the accusative case. In the sentence, via est longa,
via is the subject, and est longa is the predicate. However,
longa modifies the subject, via. Because via is nominative and the adjective
longa modifies it, it has to be nominative to agree in case, number, and gender.