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 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.