The Stem of nouns of the Second Declension ends in ŏ-: as, viro- (stem of vir, man), servo- (stem of servus or servos, slave), bello- (stem of bellum, war).
The Nominative is formed from the stem by adding s in masculines and feminines, and m in neuters, the vowel ŏ being weakened to ŭ (see §§ 6. a, 46. N.1).
In most nouns whose stem ends in rŏ- the s is not added in the Nominative, but o is lost, and e intrudes before r,
if not already present: as, ager, stem agrŏ-
; cf. puer, stem puero-.
Exceptions:erus, hesperus, iūniperus, mōrus, numerus, taurus, umerus, uterus, vīrus, and many Greek nouns.
The stem-vowel ŏ has a variant form ĕ,
which is preserved in the Latin vocative singular of nouns in -us: as, servĕ, vocative of servus, slave.
Nouns ending in -us (-os), -er, -ir, are Masculine; those ending in -um (-on) are Neuter.
Exceptions: Names of countries and towns in -us (-os) are Feminine: as, Aegyptus, Corinthus. Also many names of plants and gems, with the following: alvus, belly;carbasus, linen (pl. carbasa, sails, N.); colus, distaff;humus, ground:vannus, winnowing-shovel.
Many Greek nouns retain their original gender: as, arctus (F.), the Polar Bear; methodus (F.), method.
The following in -us are Neuter; their accusative (as with all neuters) is the same as the nominative: pelagus, sea;vīrus, poison;vulgus (rarely M.), the crowd. They are not found in the plural, except pelagus, which has a rare nominative and accusative plural pelagē.
a. The Locative form of this declension ends for the singular in -ī: as, humī, on the ground;Corinthī, at Corinth; for the plural, in -īs: as, Philippīs, at Philippi (cf. p. 34, footnote).
The genitive of nouns in -ius or -ium ended, until the Augustan Age, in a single -ī: as, fīlī, of a son;Pompêī, of Pompey (Pompêius); but the accent of the nominative is retained: as, ingĕ'nī, of genius.
Proper names in -ius have -ī in the vocative, retaining the accent of the nominative: as, Vergĭ'lī. So also, fīlius, son;genius, divine guardian: as, audī, mī fīlī, hear, my son.
Adjectives in -ĭus form the vocative in -ie, and some of these are occasionally used as nouns: as, Lacedaemonie, O Spartan.
The genitive plural often has -um or (after v) -om (cf. § 6. a) instead of -ōrum, especially in the poets: as, deum, superum, dīvom, of the gods;virum, of men. Also in compounds of vir, and in many words of money, measure, and weight: as, Sēvirum, of the Seviri;nummum, of coins;iūgerum, of acres.
The original ending of the ablative singular (-ōd) is sometimes found in early Latin: as, Gnaivōd (later, Gnaeō), Cneius.
Proper names in -âius, -êius, -ôius (as, Aurunculêius, Bôī), are declined like Pompêius.
The following stems in ero-, in which e belongs to the stem, retain the e throughout and are declined like puer (§ 47):—
adulter, adulterer;
gener, son-in-law;
puer, boy;
socer, father-in-law;
vesper, evening;
Līber, Bacchus.
Also, the adjective līber, free, of which līberī, children, is the plural (§ 111. a), and compounds in -fer and -ger (stem fero-, gero-): as, lūcifer, morning star;armiger, squire.
An old nominative socerus occurs. So vocative puere, boy, as if from †puerus (regularly puer).
Vir, man, has genitive virī; the adjective satur, sated, has saturī; vesper, evening, has ablative vespere (locative vesperī, in the evening).
Mulciber, Vulcan, has -berī and -brī in the genitive. The barbaric names Hibēr and Celtibēr retain ē throughout.