The Uses of the Genitive Case
What is special about the genitive case is that it lets one noun modify another noun or show a close link between two nouns without saying that they are the same thing. For instance, “Chris's truck” shows a close link between Chris and a truck, but it doesn't say that they are the same thing. This example shows possession, but the genitive case shows more than just possession. If you refer to “a tablespoon of vinegar,” “tablespoon” and “vinegar” certainly aren't the same thing, nor does the tablespoon own the vinegar. There is merely a close link between those two nouns.
You can link two nouns and mean that they are the same thing. This is called apposition. All you have to do is put the second noun in the same case as the first. For example, to say “I saw your mother, Aurelia, yesterday” would be herī matrem tuam Aureliam vī dī
The most common — and most obvious — use of the genitive case is to show possession. English has two ways to do this. One is through English's own genitive case endings (i.e., “-'s” or “-s'”). The other is with the preposition “of.” “The farmer's fields” means the same as “the fields of the farmer.”
Iter Marcī erat longum et difficile.
Like most modifiers in Latin, words in the genitive case tend to follow the nouns they modify.
The Latin words meus (“my”), tuus (“your”), noster (“our”), vester (“your”), suus (“his, her, its, their”) are possessive adjectives. They already have the idea of possession in their meanings, and being adjectives, they must agree in gender, case, and number with the noun they modify; for example, canis meus (my dog), canēs meī (my dogs).
Another common use of the genitive case is called the partitive genitive. This use shows something of which a part is taken. For example, in the phrase “a cup of sugar,” “sugar” would be in the genitive because it is the something of which a “cup” is taken.
Pars exercitū s extrā urbem manē bat.
At times, what is partitive in English is not partitive in Latin and vice versa. In English you can say “all the people” or “all of the people” and it means the same thing. In Latin, you could say only omnēs hominē s. On the other hand, the Latin word nihil (nothing) uses a partitive genitive, so nihil cibī literally translates as “nothing of food,” meaning “no food.”
Watch out for the comparative of the adjective multus! In the singular, plū s is a third declension neuter noun (plū s, plū ris, n. — more) and always takes a partitive genitive. Plūs cibī literally says “more of food” meaning “more food.” The English word “more” can use a partitive or not; in Latin it is always partitive.
Subjective and Objective Genitives
The subjective and objective genitive case uses can be tricky. These two genitive case uses both involve a noun in the genitive paired with a noun that has a verbal idea like “love.” “Love” can be a noun or a verb. More often than not, however, the noun and the verb are closely related, but not identical. Take for instance the word “growth.” It is a noun, but it is clearly related to the verb “grow.”
The terms “subjective” and “objective” have something to do with verbs as well, don't they? Subjects perform actions, objects receive them. Consider this sentence:
Augur mī tum avium in caelī spectā bat.
The genitive phrase is mī tum avium (“the movement of birds”). The birds don't own movement. Movement is not a quantity of birds. But the birds are moving — they are the subjects of the verbal idea of the noun mī tum (“movement”). Avium, then, is a subjective genitive.
Now we can turn this around. Here is another sentence:
Odium bellī in oppidī erat magnum.
Odium (“hatred”) has an obvious link to the verb “to hate.” But is the war doing the hating, or is war what is hated? In this example, the word in the genitive is receiving the action. That is to say, it is the object. Bellī, then, is an objective genitive.
Genitive of Description
A noun in the genitive case can be used to describe another noun.
Melissa erat mulier animī bonī.
The only restriction in this construction is that the noun in the genitive must be accompanied by an adjective, just as bonī agrees with animīin the example above.
As an alternative to a noun with an adjective in the genitive, you can use the ablative case without a preposition instead. Melissa erat mulier animō bonō.The meaning is exactly the same; the only difference is stylistic. This ablative use is predictably called the Ablative of Description.