Introduction
Fourth Declension Nouns: These nouns are mostly masculine, with some feminine exceptions like manus (hand). They often have a characteristic -us ending in the Nominative singular, similar to second declension masculine nouns, but with -s-, -t-, or -x- before the -us.
Forming Fourth Declension Nouns: You form these nouns by adding endings to the word stem. For example:
gradus (grade, step)
arcus (arch)
sinus (cavity)
Feminine Exceptions: Some feminine nouns in the fourth declension don't have the characteristic consonants s, t, x before -us. These include:
acus (needle)
manus (hand)
Masculine Exceptions: Some masculine nouns also lack these characteristic consonants:
arcus (arch)
sinus (cavity)
Other Masculine Examples:
gustus (taste)
auditus (hearing)
tactus (sense of touch)
olfactus (smell), synonym: odoratus
Neuter Gender: Neuter fourth declension nouns have a -uum ending in the genitive plural, similar to the third declension but with two "u"s. They also have the typical -a ending in the nominative and accusative plural. Examples include:
cornu (horn)
genu (knee)