Ancient Greek/Basic Nouns/Second Declension
The second declension, in contrast to the first, consists primarily of masculine and neuter nouns. It is occasionally referred to as the ο-declension, because of the recurrence of the vowel omicron. It is the simplest of the three declensions of Ancient Greek, featuring a bit more than a single set of endings, and regular persistent accentuation throughout.
As in the first declension, the declension remains identical in the nominative, vocative, and accusative of the dual. Moreover, after one recognizes the basic second declension (which in many ways is similar to the first declension), the accentuation is similar to that of the first declension: ἄνθρωπος like ὕγίεɩᾰ; θεός and πτερόν like θεᾱ́ or ἡδονή.
Masculine and feminine nouns
These can be recognized by their -ος ending, which was transliterated as -us in Latin and found its way into English (e.g., "Dionysus"). The vast majority of these are masculine.
ἄνθρωπος, ἀνθρώπου, (ánthrōpos, anthrópou) m, "man; human being"
The nominative and vocative plurals are always identical. Note that the lengthening of the ending causes the accent to shift (as the accent cannot stand on the antepenult if the ultima is long) in several of the cases. Also recall that final -οι is usually short.
θεός, θεοῦ, (theós, theoû) m, "god"
νῆσος, νήσου, (nêsos, nésou) f, "island"
Again, note the persistent accent, which changes to an acute before long endings, as expected.
These end in -ον and have the nominative, accusative, and vocative identical in both the singular and the plural, which is a feature of all neuter nouns in Ancient Greek:
πτερόν, πτεροῦ, (pterón, pteroû) n, "wing"
Note that, except in the nominative, accusative, and vocative, the endings are identical to those of masculine and feminine nouns of the second declension. Also note that, just as in the first declension, whenever a noun of the second declension is accented on the ultima, the accent changes to a circumflex in the genitive and dative singular and plural.