4thdeclension

The fourth declension is a group of Latin nouns that share a characteristic set of endings, including a genitive plural in -uum. It contains mainly masculine and feminine nouns, with a smaller number of neuter nouns. The forms can be irregular in practice, but several core patterns recur across the class.

In the masculine and feminine subset, the nominative singular typically ends in -us, and the other singular forms follow a pattern around -ūs for the genitive, -u for the dative and ablatives, and -um for the accusative. The plural normally shows nom -ūs, gen -uum, dat -ibus, acc -ūs, and abl -ibus. A classic example is exercitus, with singular forms such as exercitus (nom), exercitūs (gen), exercitū (dat/abl), and exercitum (acc); plural forms include exercitūs (nom), exercituum (gen), exercitibus (dat/abl), and exercitus (acc). The noun manus (hand) is a well-known irregular member of this declension, having genitive plural manuum and dative/ablative plural manibus.

Neuter nouns of the fourth declension behave similarly in number but differ in case endings. Singular nominative and accusative end in -u, while genitive is -ūs and the dative/ablative is -u. Plural nominative and accusative end in -ua, genitive is -uum, and dative/ablative is -ibus. Examples include cornu (horn) and genu (knee), both neuter with singular -u forms and plural -ua.

In usage, fourth-declension nouns cover a range of semantic fields, including body parts (manus), military terms (exercitus), and political entities (senatus). This declension is taught with attention to its key endings and the few notable irregularities.