Declining Cardinal Numbers

Only three of the cardinal numbers are fully declined: unus, duo, tres. The cardinal number mille (1000), has a related noun form which can be declined: milia, milium, n. pl - thousands. All the remaining cardinal numbers do not decline (the term for an adjective or noun that cannot be declined is indeclinable). Study the declension charts for unus, duo, tres and mille below.

Unus

Only uses singular forms; note the genitive (-ius) and dative (-i) forms (the pattern you see for unus is used for all irregular adjectives).

**Declension of unus, una, unum - one**
Case	Masculine	Feminine	Neuter
Nominative	unus	una	unum
Genitive	unius	unius	unius
Dative	uni	uni	uni
Accusative	unum	unam	unum
Ablative	uno	una	uno
Vocative	une	una	unum

Duo

Only uses plural forms; note the irregular looking dative and ablative forms. We have actually seen the ending -abus before: 1st declension nouns like filia or dea, which use the same noun base as a 2nd declension masculine noun (filius, deus) use -abus for the dative and ablative plural to distinguish it from the masculine -is (filiabus vs. filiis). The ending -obus is just a masculine/neuter version of -abus, used exclusively for the adjective duo.

**Declension of duo, duae, duo - two**
Case	Masculine	Feminine	Neuter
Nominative	duo	duae	duo
Genitive	duorum	duarum	duorum
Dative	duobus	duabus	duobus
Accusative	duos	duas	duo
Ablative	duobus	duabus	duobus
Vocative	duo	duae	duo

Tres

Only uses plural forms; declined following 3rd declension i-stem rules. Note that the masculine and feminine forms are identical, so the dictionary entry is often written: tres, tria - three. Here, we will expand it to show all three genders, for clarity.

**Declension of tres, tres, tria - three**
Case	Masculine	Feminine	Neuter
Nominative	tres	tres	tria
Genitive	trium	trium	trium
Dative	tribus	tribus	tribus
Accusative	tres	tres	tria
Ablative	tribus	tribus	tribus
Vocative	tres	tres	tria

Mille

The number 1000 is indeclinable (is not declined) as an adjective, but has a declinable noun form, which uses neuter plural, 3rd declension i-stem forms:

**Declension of mille - 1000**
Case	Adjective	Plural Noun	Noun Meaning
Nominative	mille	milia	Thousands...
Genitive	mille	milium	of thousands
Dative	mille	milibus	to/for thousands
Accusative	mille	milia	...thousands
Ablative	mille	milibus	by/with/from thousands
Vocative	mille	milia	thousands (being addressed)