Latin declension

Latin declension is the set of patterns in the Latin language for how nouns and certain other parts of speech (including pronouns and adjectives) change form according to their grammatical case, number and gender. Words that change form in this manner are said to be declined.

Declension is normally marked by suffixation: attaching different endings to the declined word. For nouns, Latin grammar instruction typically distinguishes five main patterns of endings. The patterns are numbered from first to fifth and subdivided by grammatical gender. The term "declension" can refer either to the overarching phenomenon, or to one of these specific five patterns. For example, nouns that have a genitive singular form that ends in -ae are said to belong to "the first declension".

Adjectives are of two kinds: those like bonus, bona, bonum 'good' use first-declension endings for the feminine, and second-declension for masculine and neuter. Other adjectives such as celer, celeris, celere belong to the third declension. There are no fourth- or fifth-declension adjectives.

Pronouns are also of two kinds, the personal pronouns such as ego 'I' and tū 'you (sg.)', which have their own irregular declension, and the third-person pronouns such as hic 'this' and ille 'that' which can generally be used either as pronouns or adjectivally. These latter decline in a similar way to the first and second noun declensions, but there are differences; for example the genitive singular ends in -īus or -ius instead of -ī or -ae and the dative singular ends in -ī.

The cardinal numbers ūnus 'one', duo 'two', and trēs 'three' also have their own declensions (ūnus has genitive -īus and dative -ī like a pronoun). However, numeral adjectives such as bīnī 'a pair, two each' decline like ordinary adjectives.

Declension is a specific type of inflection, and is distinguished from other ways that words change form in the Latin language, such as the conjugation of verbs.