Latin Nouns of the Second Declension Endings

The second declension is characterized by an "-o". This is the declension you would use if you wanted to decline the name Aurelius as in Marcus Aurelius*.

Second declension nouns in Latin are mostly masculine or neuter, but there are also feminine nouns that are declined like masculine ones.

The nominative of neuter nouns will always be the same as the accusative. The singular nominative/accusative second declension noun ends in "-um." Regardless of the declension, the ​plural neuter nominative and accusative always ​ends in "-a." If you study Greek, you will find this alpha ending in the neuters there, as well.

While first declension nouns end in "-a", second declension nouns (masculine, since we've dispensed with neuters) usually end in "-us," "-ius," or "er." Other second declension endings for the nominative are "ir," "ur," "os," "on," and "um." Greek-based "Pelion" and "Andros" are examples of the second declension nouns ending in "os" and "on." If the nominative ends in "-us," you simply drop the ending and replace it with the "-i" for the genitive. You do the same for the "-ius" ending, but notice that you now have a double "i". If the noun ends in "-er," you really need to see the dictionary or lexicon for the genitive to know how to decline the noun:

Puer, Latin for boy, adds the endings to puer, but cancer, Latin for crab, does not. The genitive of cancer is cancri. The "e" has dropped out. The dictionary entry for the two nouns should be something like:

• puer, -i m., boy
• cancer, -ri m., crab

The endings of the second declension are:
singular
nominative -us
genitive -i
dative -o
accusative -um
ablative -o

plural
nominative -i
genitive -orum
dative -is
accusative -os
ablative -is

Sample Declension of a 2nd Declension Masculine Noun: Somnus, - i, m. 'Sleep'

singular

• Nominative - somnus
• Genitive - somni
• Dative - somno
• Accusative - somnum
• Ablative -somno
• Locative - somni
• Vocative - somne

plural

• Nominative - somni
• Genitive - somnorum
• Dative - somnis
• Accusative - somnos
• Ablative - somnis
• Locative - somnis
• Vocative - somni

* For the name Marcus Aurelius, you might decline it thus:
M. Aurelius, M. Aurelii, M. Aurelio, M. Aurelium, M. Aurelio. Since Marcus Aurelius is one person, you would be very unlikely to decline his name in the plural.