Ancient Greek/Basic Nouns/First Declension
The first declension consists primarily of feminine nouns, with a few masculines. It is characterized by the recurrence of the letter alpha, and for this reason it is often referred to as the alpha declension, although the alpha was often changed to eta in the Attic dialect.
Nouns of the first declension have persistent accents. This means that, in general, when these nouns are declined, the accent does not move, although it does move if not doing so would violate one of the rules of accentuation, and it changes from an acute to a circumflex or vice versa if required by those rules. (This can happen when the final syllable goes from being short to long or vice versa; note that the terminal αι in the nominative plural is considered short.) In addition, when the accent in the nominative falls on the last syllable, it changes to a circumflex in the genitive and dative singular and plural.
The accent on the genitive plural of a first declension noun always falls on the last syllable.
Feminine nouns of the first declension may be further subdivided on the basis of α/η and ᾱ/ᾰ distinctions in the singular. However, in the plural, all nouns of the first declension have the same endings, which consistently have alpha instead of eta. The nominative singular and vocative singular are always identical, as are the nominative plural and vocative plural.
Nouns in eta
In Attic Greek, most feminine nouns of the first declension have eta throughout the singular, because long alpha was usually changed to eta in this dialect, except after epsilon, iota, or rho.
ἡδονή, ἡδονῆς, "pleasure"
νίκη, νίκης, "victory"
Note: There is nothing irregular about the accentuation of νῑ́κη. In the nominative and vocative plural, the last syllable becomes short, which forces the acute to change to a circumflex, per the rules of accentuation.
Nouns in long alpha
After epsilon, iota, or rho, the long alpha is retained:
θεᾱ́, θεᾶς, "goddess"
χώρᾱ, χώρᾱς "land, country"
δημοκρατίᾱ, δημοκρατίᾱς, "democracy"
Nouns in short alpha
Like the more common nouns in long alpha, these have stems ending in epsilon, iota, or rho. The short alpha appears in the nominative, accusative, and vocative singular:
ὑγίειᾰ, ὑγιείᾱς "health"
Note the shift in accent resulting from the lengthening of the ending.
Nouns in short alpha and eta
These nouns generally had stems ending in -σσ-, which was changed in Attic to -ττ-. They have short alpha in the nominative, accusative, and vocative singular, and eta in the genitive and dative singular.
θάλαττᾰ, θαλάττης, "sea"
Masculine nouns of the first declension end in -ης or -ᾱς, with the -ου ending characteristic of the second declension in the genitive singular. Other than that their case endings are the same as those of feminine nouns of the first declension, although the ending of the vocative singular is inconsistent (and may be -ᾱ, -ᾰ, or -η). Note that the vocative plural, however, still matches the nominative plural.
νεᾱνίᾱς, νεᾱνίου, "young man"
πολῑ́της, πολῑ́του, "citizen"