45. The Stem of nouns of the 2nd Declension ends in -ŏ.
viro- (stem vir man)
servo- (stem servus or servos slave)
bello- (stem bellum war)
a. 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 and § 46. Note 1).
b. In most nouns whose stem ends in strong -rŏ- the s is not added in the Nominative, but o is lost, and e is added before r,1 if not already present.
ager, stem agrŏ-2
cf. puer, stem puero-
Exceptions: erus, hesperus, iūniperus, mōrus, numerus, taurus, umerus, uterus, vīrus, and many Greek nouns.
c. The stem vowel -ŏ has a variant form -ĕ,3 which is preserved in the Latin vocative singular of nouns in -us.
servĕ, vocative of servus slave
Note— In composition this -ĕ appears as -ĭ. Thus:
belli-ger warlike (from bello/e-, stem bellum war).
46. Nouns of the 2nd declension in -us (-os) and -um (-om) are thus declined.
Note 1— The earlier forms for nominative and accusative were -os, -om, and these were always retained after u- and v- up to the end of the Republic. The terminations -s and -m are sometimes omitted in inscriptions.
Cornēlio for Cornēlios, Cornēliom
Note 2— Stems in quo-, like equo-, change qu to c before u. Thus, ecus (earlier equos), equī, equō, ecum (earlier equom), eque. Modern editions disregard this principle.
47. Nouns of the 2nd declension in -er and -ir are thus declined.
Note— When e belongs to the stem, as in puer, it is retained throughout; otherwise it appears only in the nominative and vocative singular, as in ager.
48. 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.
Also, many names of plants and gems, as well as:
carbasus linen [plural carbasa (n.) sails]
Many Greek nouns retain their original gender.
arctus (f.) the Polar Bear
methodus (f.) method
a. The following in -us are neuter; their Accusative (as with all neuters) is the same as the Nominative.
vulgus (rarely m.) the crowd
They are not found in the plural, except pelagus, which has a rare Nominative and Accusative plural pelagē.
Note— The nominative plural neuter cētē, sea monsters, occurs; the nominative singular cētus occurs in Vitruvius.
1. Compare the English chamber from French chambre.
2. Compare Greek ἀγρός, which shows the original o of the stem.