How a noun changes its endings depends on the declension to which it belongs. There are five declensions. Below are examples of nouns belonging to that declension and how they decline.
Model noun – mensa, -ae (f), ‘table’.
Nouns of the 1st declension end in –a and are usually feminine. (Notable exceptions are: nauta, sailor; agricola, farmer.)
Model noun – dominus, i (m), master.
Nouns of the 2nd declension end in –us and are usually masculine.
- Variant: second declension nouns ending in -r.
Model noun – puer, pueri (m), boy.
Note: for nouns with a consonant before the ‘e’ in ‘er’, the ‘e’ is dropped. Examples: magister, magister, magistrum etc.; ager, ager, agrum etc.
- Variant: second declension neuter
Model noun – bellum, -i (n), war.
Essentially, these are the same as normal 2nd declension nouns except for the nominative, vocative and accusative, which are identical in singular (all –um) and plural (all –a).
Model noun – rex, regis (m), king.
The nominative and vocative singular of 3rd declension nouns do not follow any pattern at all. The endings are added onto the stem’ of the noun. This is found by taking the genitive singular and taking off the ‘is’ at the end. The endings are boldened in the table. In a dictionary, 3rd declension nouns are given with their nominative and genitive forms so that you may find the stem.
Nouns of the 3rd declension can be of any gender. There is no particular reason for a 3rd declension noun being masculine or feminine, but there are some that are obvious, e.g. ‘mater’ (mother) is feminine while ‘pater’ (father) is masculine. Nouns ending in ‘as’, e.g. ‘tempestas’ (storm) are usually feminine.
- Variant: third declension neuter
Model noun – litus, litoris (n), shore.
Essentially these are the same as other 3rd declension nouns apart from the nominative, vocative and accusative, which are identical in singular (all the same as the nominative) and plural (all –a).
Equilibrium is a minor point that affects the genitive plural of nouns. If the nominative and genitive forms of the noun have the same number of syllables, that means they have ‘equilibrium’. If they have equilibrium, then the genitive plural ends in ‘-ium’ as opposed to ‘um’. There are 2 groups of exceptions: the family (mother, father, brother, old man, young man, dog), and monosyllabic words whose stem ends in two consonants e.g. (nox, noctis – the stem is noct).