We will now complete the table of nouns with the 3rd, 4th, and 5th declensions. These declensions are more difficult to work with because their nominative and accusative plural forms are identical, as are their dative and ablative plural forms. To distinguish the cases, you must use a very simple key: context. Context will tell you the meaning.
3rd declension nouns have two stems: The nominative and vocative singular stem and the stem used for all other cases. Both stems have to be memorized for each noun. Feminine and masculine forms are indistinguishable.
3rd Declension Masculine or Feminine, no i-stem: (each word has a set gender): rēx, m.[edit | edit source]
3rd Declension Masculine or Feminine, 2-consonant base i-stem: (each word has a set gender): ars, artis, f.[edit | edit source]
i-stem nouns differ from other 3rd declension nouns in that some of the forms have endings changed to include is.
There are two main kinds of masculine/feminine i-stem nouns. The first kind has its usual stem end in two consonants; the example here, for instance, has its base art- end in -rt-. The last consonant of the nominative singular form always ends in either -s or -x.
3rd Declension
Singular
Plural
nominative
ars
art-ēs
accusative
art-em
art-ēs
genitive
art-is
art-ium
dative
art-ī
art-ibus
ablative
art-e
art-ibus
3rd Declension Masculine or Feminine, parisyllabic i-stem: (each word has a set gender): nūbēs, f.[edit | edit source]
The other kind of masculine/feminine i-stem noun has the property that its nominative and genitive singular forms have the same number of syllables. They are therefore called parisyllabic. All nouns of this form have their nominative singular form end in either -ēs or -is.
Neuter i-stem nouns have their nominative singular forms end with -al, -ar, or -e.
3rd Declension Neuter
Singular
Plural
nominative*
mare
mar-ia
accusative
mare
mar-ia
genitive
mar-is
mar-ium
dative
mar-ī
mar-ibus
ablative
mar-ī
mar-ibus
List of common 3rd declension stem change patterns[edit | edit source]
Singular Nominative
Main stem
Main gender
Examples
-is
-is
masc/fem
canis, navis, hostis
-s
-is
masc/fem
urbs, rex*, matrix*
-s
-tis
masc/fem
nox*, mons, pons
-o
-onis
masc/fem
legio, auditio, statio
-en
-inis
neuter
carmen, flumen, examen
-or
-oris
masc/fem
amor, timor
-us
-oris
neuter
litus, corpus
-us
-eris
neuter
genus, vulnus
regs and matrics, respectively, but the gs and cs both compound into x. The c and g stay in the other cases, hence regis and matricis as their genitives. Nox (gen. noctis) works similarly.