RULES.—a. If the nominative singular ends in alpha preceded by a vowel (σκιά_
b. If the nominative singular ends in alpha preceded by a consonant not ρ,
c. If the nominative singular ends in η, η is kept in all the cases of the singular.
d. When the genitive singular has -ης, final α of the nominative singular is always short; when the genitive singular has -α_ς, the final α is generally long.
Feminines fall into two classes:
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].