a. Observe that the masculines and the neuters have the same terminations excepting in the nominative singular and the nominative and accusative plural.
b. The vocative singular of words of the second declension in -us ends in -ĕ, as domine, O master; serve, O slave. This is the most important exception to the rule in §56. a.
74. Write side by side the declension of domina, dominus, and pīlum. A comparison of the forms will lead to the following rules, which are of great importance because they apply to all five declensions:
a. The vocative, with a single exception (see §73. b), is like the nominative. That is, the vocative singular is like the nominative singular, and the vocative plural is like the nominative plural.
b. The nominative, accusative, and vocative of neuter nouns are alike, and in the plural end in -a.
c. The accusative singular of masculines and feminines ends in -m and the accusative plural in -s.
d. The dative and ablative plural are always alike.
e. Final -i and -o are always long; final -a is short, except in the ablative singular of the first declension.
75. Observe the sentences
Lesbia est bona, Lesbia is good
Lesbia est ancilla, Lesbia is a maidservant
We have learned (§55) that bona, when used, as here, in the predicate to describe the subject, is called a predicate adjective. Similarly a noun, as ancilla, used in the predicate to define the subject is called a predicate noun.
76. Rule. Predicate Noun. A predicate noun agrees in case with the subject of the verb.