Nouns are used in several different ways, such as to show possession or as the subject of a sentence. When a person takes a noun and lists all of its different forms according to how the noun is used, it is called a declension. In Latin, however, each noun can have seven singular and seven plural forms, and the rules governing the endings used determine to which declension the noun belongs. First declension nouns are normally feminine, second declension nouns are typically masculine or neuter and third declension nouns may be feminine, masculine or neuter. Third declension nouns also require greater alterations to the root word prior to the addition of the endings that show case and number.
Root words for third declension nouns may have consonant stems or i-stems. Examples of consonant stem nouns include rex, mater, and opus, which mean king, mother and work, respectively. The i-stem nouns include mare, hostis, and animal, which may be translated as sea, enemy and animal, respectively. To understand the distinction, an abbreviated overview of the cases for nouns may be helpful.
The function of a noun within a sentence determines its case. Latin uses seven different cases, which are the nominative, genitive, dative, accusative, ablative, locative and vocative cases. The nominative case is sometimes called the naming or subject case, and it is the first form of the noun listed in a dictionary. To illustrate, rex, mater, opus, mare, hostis and animal are all in the nominative case. Possession is shown through the use of the genitive case, and the proper genitive case for a Latin noun is shown as the second word in a dictionary.
Genitive case for third declension nouns often represents a significant change from the nominative. For example, rex becomes regis,opus becomes operis, mare becomes maris and animal becomes animalis. Once the genitive singular form is known, it is relatively simple to form the other cases.
For third declension consonant stem singular cases, drop the -is from the genitive singular and add -i, -em, or –e to form the dative, accusative and ablative singular cases, respectively. Plural case endings for the nominative and accusative cases depend on whether the noun is neuter; add -es for feminine and masculine forms and -a for neuter forms in both the nominative and accusative plural. The respective endings for the plural genitive, dative and ablative cases are the same, regardless of gender. Add -um for genitive and -ibus for both the dative and ablative cases.